I've successfully plotted the accounts on Google Map with Lightning Component and it works in Sandbox...but don't know how to write a test code for the ApexClass.

I describe the codes below and hope anyone can help with the test code part. Thank you!

Component (MapNearbyAccount.cmp)

<aura:component controller="MapNearbyAccountController" implements="flexipage:availableForAllPageTypes,force:hasRecordId">
    <aura:attribute name="mapMarkers" type="Object"/>
    <aura:attribute name="selectedMarkerValue" type="String" />
    
    <aura:handler name="init" value="{! this }" action="{! c.init }"/>
    
    <div class="slds-box slds-theme--default">
        <lightning:map 
                       mapMarkers="{! v.mapMarkers }"
                       selectedMarkerValue="{!v.selectedMarkerValue}"
                       markersTitle="accounts nearby"
                       listView="auto"
                       showFooter="false"
                       onmarkerselect="{!c.handlerMarkerSelect}" />
    </div>
</aura:component>

Controller (MapNearbyAccount.js)

({
    init: function (cmp, event, helper) {
        var recordId = cmp.get("v.recordId");     
        var action = cmp.get("c.getAccounts");
        action.setParams({recordId :recordId});
        cmp.set('v.mapMarkers', [{location: {}}]);

        action.setCallback(this, function(response){
            
            var accounts = response.getReturnValue();
            var markers = [];
            for(var i = 0; i < accounts.length; i++){
                var acc = accounts[i];
                markers.push({
                    location: {
                        Country : acc.BillingCountry,
                        State : acc.BillingState,
                        City: acc.BillingCity,
                        Street: acc.BillingStreet
                    },
    
                    icon : "standard:account",
                    value: acc.Id,
                    title: acc.Name,
                    description:acc.Description
                });
            }
            
            if(markers.length != 0){
                cmp.set('v.mapMarkers', markers);
            }
        });

        $A.enqueueAction(action);
    },

    handlerMarkerSelect: function (cmp, event, helper) {
        console.log(event.getParam("selectedMarkerValue"));
    }
});

ApexClass (MapNearbyAccountController)

public class MapNearbyAccountController {
    @AuraEnabled
    public static List<Account> getAccounts(String BillingCity, String BillingState, String recordId){
        Account acct = [SELECT Id, Name, BillingCountry, BillingState, BillingCity, BillingStreet, Industry FROM Account WHERE Id =:recordId];
        
        return [SELECT Id, Name, BillingCountry, BillingState, BillingCity, BillingStreet,Description
                FROM Account 
                WHERE BillingState = :acct.BillingState AND BillingCity LIKE :('%' + acct.BillingCity + '%') AND Industry = :acct.Industry LIMIT 10];
    }
}

TestClass

@isTest
public class MapNearbyAccountControllerTest {
@isTest
    static void testMapNearbyAccountController() {
        Account acc1 = new Account();
        acc1.Name='acc1';
        acc1.BillingCity='Shibuya';
        acc1.BillingState='Tokyo';
        insert acc1;
        
        MapNearbyAccountController ctrl = new MapNearbyAccountController();
        
        Test.startTest();
            List<Account> getAccounts = ctrl.getAccounts();
            System.assertEquals(false,getAccounts.isEmpty());
        Test.stopTest();
    }
    
}