Check process start command for successful buildpack_cache usage instead

[sethboyles]

Are you submitting this PR against the develop branch?

yes

What is this change about?

This test verifies that buildpacks can successfully use the buildpack cache.

Currently it does this by pushing and curling a one-line bash app. Recently, it's been flaky. Here are a few examples of flakes (though there are many others):

https://concourse.wg-ard.ci.cloudfoundry.org/teams/main/pipelines/cf-deployment/jobs/experimental-cats-cflinuxfs4/builds/168
https://ci.capi.land/teams/main/pipelines/capi/jobs/leia-azure-storage/builds/976

The issue is the start command seems to be set correctly, and the app is returning 502's/404's other errors for some reason. However, this test is not about how an app performs when running, it's about the staging/compilation process, so we opted to just check the process's start command for the same information.

What version of cf-deployment have you run this cf-acceptance-test change against?

Please check all that apply for this PR:

• introduces a new test --- Are you sure everyone should be running this test?
• changes an existing test
• requires an update to a CATs integration-config

Did you update the README as appropriate for this change?

• YES
• N/A

If you are introducing a new acceptance test, what is your rationale for including it CATs rather than your own acceptance test suite?

N/A

How many more (or fewer) seconds of runtime will this change introduce to CATs?

What is the level of urgency for publishing this change?

• Urgent - unblocks current or future work -- we are blocked on releasing CAPI because this flakes in our pipeline often
• Slightly Less than Urgent

Tag your pair, your PM, and/or team!

@xandroc

[tjvman]

We've seen a similar failure occur in apps/app_stack.go, which uses a similar custom buildpack approach.

[tjvman]

The following files contain similar custom buildpack + nc -l logic, and may be similarly flaky:

apps/admin_buildpack_lifecycle.go
apps/app_staging_environment.go
credhub/service_bindings.go
v3/buildpacks.go