Hello,

Note that car manufacturers uses several tests before implementing a software in prototype vehicles. The purpose of these tests is to avoid all the safety failures and most of the functional failures before driving.
When I was working for a truck constructor, there was three tests conducted before launching the production.
Each test was able to avoid a kind of failures.

1. Unitary tests.
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The purpose is to detect and correct all the software & hardware bugs due to bad coding for each ECU (for example bad memory management, bad acccuracy in a computation, stack overflow, bad hardware diagnosis, ....)
Each ECU (Electronic Control Unit) is tested individually with a test tool developped to simulate the remaining parts of the vehicle : switches for digital inputs, some actuators, CANalyser to simulate the remaining vehicle CAN bus activity, and so on...
This test is intended to be sure that the ECU will not disturb the others when the integration will be done and that it works as specified. This test allows to test easily all the code by simulating all the conditions allowing a part of code to run.
This is the test your customer want you to conduct with a good test plan and a good test report. You'll have also to explain how you've coded, which rules you've used. Sometimes on safety devices, the curstomer could force you to uses a standard method or rules.

2. Integration plateform
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The purpose is to detect and correct all the intersystem functional bugs (due to issues not identified by specifications for example)
This is a test tool with all the ECUs connected with a representative electrical vehicle architecture. A Matlab model simulates the vehicle behaviour. This is as if you were in the vehicle. You can "drive" on the simulator except you won't have to turn the steering wheel.
All the safety issues must be corrected there.
And usually you finds 99.9% of functional bugs.
You shouldn't detect an ECU issue there. All ECUs must not have any remaining design problem.
Be sure that sometimes we're still finding some design issues !

3. Vehicle prototype test
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Well, now the prototype is built. You will have to test all is correct and often adjust some parameters.
No functional bugs should be detected there, but you know .... test plans are not perfects !

Next, there is an other phase allowing to test the whole manufacturing process by manufacturing many vehicles as it was a full production.

And then the first production vehicle is built !
You are proud to have contributed toward a such project... Except if a problem occurs in field and if you're directly involved. But it's an other story !

regards
Stephane