I'm trying to build a VF Page with a standard controller "Opportunities" and extension that can update fields from Accounts.

This is my VF Page:

<apex:page standardcontroller="Opportunity" sidebar="true">
    <apex:form >
     <apex:pageblock >
      <apex:commandButton value="Save" action="{!save}" rerender="error"/>
      <apex:commandButton value="Quick Save" action="{!quicksave}" rerender="error"/>
      <apex:commandButton value="Cancel" action="{!cancel}" rerender="error"/>
      </apex:pageblock>
      <apex:pageBlock mode="edit">
          <apex:pageBlockSection columns="2" showHeader="true" title="{!opportunity.name}">
              <apex:inputField value="{!Opportunity.Schedule_Type__c}" required="false"/>
              <apex:inputField value="{!Opportunity.LOI_In__c}" required="false"/>
              <apex:inputField value="{!Opportunity.Amount}" required="false"/>
              <apex:inputField value="{!Opportunity.Schedule_Amount__c}" required="false"/>
              <apex:inputField value="{!Opportunity.Deposit_Amount__c}" required="false"/>
              <apex:inputField value="{!Opportunity.Account.SicDesc}" required="false"/>
              <apex:inputField value="{!Opportunity.Account.Business_Start_Date__c}" required="false"/>
              <apex:inputField value="{!Opportunity.Account.AnnualRevenue}" required="false"/>
              <apex:inputField value="{!Opportunity.Account.Tangible_Net_Worth__c}" required="false"/>
              <apex:inputField value="{!Opportunity.Account.EBITDA__c}" required="false"/>
              <apex:inputField value="{!Opportunity.Lease_Term__c}" required="false"/>
              <apex:inputField value="{!Opportunity.Base_Lease_Term_Implicit_Interest__c}" required="false"/>
              <apex:inputField value="{!Opportunity.Lease_Type__c}" required="false"/>
              <apex:inputField value="{!Opportunity.Equipment_Location__c}" required="false"/>
              <apex:inputField value="{!Opportunity.account.SP_Rating__c}" required="false"/>
              <apex:inputField value="{!Opportunity.Account.SP_Outlook__c}" required="false"/>
              <apex:inputField value="{!Opportunity.Risk_Rating__c}" required="false"/>
              <apex:inputField value="{!Opportunity.Residual_Ceiling__c}" required="false"/>
              <apex:inputField value="{!Opportunity.Residual_Floor__c}" required="false"/>
              <apex:inputField value="{!Opportunity.LTH_Recommendation__c}" required="false"/>
              <apex:inputField value="{!Opportunity.Most_Recent_Audit__c}" required="false"/>
              <null>
              </null>
              <apex:inputField value="{!Opportunity.Strengths_of_Transactions__c}" required="false"/>
              <apex:inputField value="{!Opportunity.Primary_Challenge__c}" required="false"/>
         </apex:pageBlockSection>
      </apex:pageblock>
    </apex:form>
</apex:page>



I've created an apex class in sandbox (not all account fields on VF page is in apex class yet since I'm trying to test it out):

public class CreditExtension {
    public ApexPages.StandardController myAccountController {get; set;}
    public Account Account {get; set;}
    
    public ApexPages.StandardController standardOpportunityController;
    
    public CreditExtension(ApexPages.StandardController cntrl){
        
        standardOpportunityController = cntrl;
        
        Opportunity con = (Opportunity)cntrl.getRecord();
    Account = [select SP_Outlook__c from Account where id = :con.Account.id] [0];
    Account = [select AnnualRevenue from Account where id = :con.Account.id] [0];
    Account = [select SP_Rating__c from Account where id = :con.Account.id] [0];
    
    }
public pagereference doSaveAll () {
    standardOpportunityController.save();
    myAccountController.save();
    return null;
}
}


When trying to deploy it I'm getting errors saying code coverage is 0%. 

Questions: How to resolve code coverage issue, and am I on the right track here with what I'm trying to ahieve?