global class Exporter implements System.Schedulable {
    global void execute(SchedulableContext sc) {
        DoExport();
    }
    
    @future(callout=true)
    public static void DoExport(){
        List<Exporter__c> exportList = new List<Exporter__c>{};
        List<Exporter__c> exportUpdateList = new List<Exporter__c>{};
        List<Messaging.SingleEmailMessage> mails  = new List<Messaging.SingleEmailMessage>();
        Datetime dt = DateTime.newInstance(Date.today(), Time.newInstance(0, 0, 0, 0));
        Date d = Date.today();
        Date firstDate = d.toStartOfMonth();
        Date lastDay = firstDate.addDays(Date.daysInMonth(d.year(), d.month())-1);
        
        system.debug('First Day: ' + firstDate);
        system.debug('Last Day: ' + lastDay);
            
        exportList = [Select Id, Name, Report_ID__c, Attachment_Name__c, Email_Subject__c, Email_Body__c, Email_Recipients__c, Frequency__c, Weekly_Days__c, Monthly_Day__c, Last_Ran__c from Exporter__c];
        
        for(Exporter__c e : exportList){
            //Determine if Exporter record is scheduled to run today.
            Boolean process = Test.isRunningTest() ? true : false;
            
            //Test for Weekly frequency.
            process = e.Frequency__c == 'Weekly' && e.Weekly_Days__c.contains(dt.format('EEEE')) ? true : process;
            
            //Test for Monthly frequency.
            process = e.Frequency__c == 'Monthly' && (e.Monthly_Day__c == String.valueOf(d.day()) || e.Monthly_Day__c == 'last' && d == lastDay || lastDay.day() <= Integer.valueOf(e.Monthly_Day__c)) ? true : process;
                       
            //Run process if scheduled.
            if(process){
                System.debug('Starting message processing for: ' + e.Name);
                ApexPages.PageReference report = new ApexPages.PageReference('/' + e.Report_ID__c + '?csv=1');
                Messaging.EmailFileAttachment attachment = new Messaging.EmailFileAttachment();
                attachment.setFileName(e.Attachment_Name__c);
                Blob content = Test.isRunningTest() ? Blob.valueOf('UNIT.TEST') : report.getContent();
                attachment.setBody(content);
                attachment.setContentType('text/csv');
                Messaging.SingleEmailMessage message = new Messaging.SingleEmailMessage();
                message.setFileAttachments(new Messaging.EmailFileAttachment[] {attachment});
                message.setSubject(e.Email_Subject__c);
                message.setPlainTextBody(e.Email_Body__c);
                String[] emailRecipients = e.Email_Recipients__c.split(',');
                message.setToAddresses(emailRecipients);
                mails.add(message);
                System.debug('Completed message processing for: ' + e.Name);
                
                //Update Last Ran date and add to update list.
                e.Last_Ran__c = System.now();
                exportUpdateList.add(e);
            }
        }
        //Send the email messages all at once to avoid limits.
        Messaging.sendEmail(mails);
        System.debug('Send emails.');
        
        //Update Export record with Last Ran date.
        update exportUpdateList;
    }
}

Test class :-

@isTest
    private class TestExporter {
       // CRON expression: midnight on March 15. Because this is a test, job executes immediately after Test.stopTest().
       // =========================================================================
       public static String CRON_EXP = '0 0 0 06 3 ? 2019';
       static testmethod void test() {
          Exporter__c e    = new Exporter__c();
              e.Name = 'Test Exporter Record';
            e.Report_ID__c = '00O7F00000ADBGJ';
            e.Attachment_Name__c = 'Test Attachment Name';
            e.Email_Subject__c = 'Test Email Subject';
            e.Email_Body__c = 'Test Email Body';
            e.Email_Recipients__c = 'monika_shirke@abc.com';
          Insert(e);
           
          Test.startTest();
     
          // Schedule the test job
          String jobId = System.schedule('ScheduleApexClassTest',
                            CRON_EXP,
                            new Exporter());
              
          // Get the information from the CronTrigger API object
          CronTrigger ct = [SELECT Id, CronExpression, TimesTriggered,
             NextFireTime
             FROM CronTrigger WHERE id = :jobId];
     
          // Verify the expressions are the same
          System.assertEquals(CRON_EXP,
             ct.CronExpression);
     
          // Verify the job has not run
          System.assertEquals(0, ct.TimesTriggered);
     
          // Verify the next time the job will run
          System.assertEquals('2019-03-06 00:00:00',
             String.valueOf(ct.NextFireTime));
          Test.stopTest();
     
       }
    }