Add function for checking that code dirs have test

[greghaynes]

There is a known issue where directories which contain code but no tests
are skipped in coverage reports. We work around this by adding a
stub_tests.go file in these directories.

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[APPROVALNOTIFIER] This PR is NOT APPROVED

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[googlebot]

CLAs look good, thanks!

[srinivashegde86]

cc @steuhs
do we need a separate check here or this is a missing feature in the coverage tool?

[adrcunha]

I assume you intended to replace pkg by $1 here?

[adrcunha]

local please.

[adrcunha]

/test pull-knative-test-infra-build-tests

[jessiezcc]

@steuhs is solving the same problem with same solution AFAIK.

[steuhs]

For the code coverage tool, I am planning on add _test.go to all .go file on the repo checked out locally. Once that is done, it will not only solve the issue mentioned here, but also cover all .go files. For now, this solution is a doable approach, but it will become redundant after the code coverage feature is updated accordingly.

[adrcunha]

@steuhs can we close this PR now that your solution is implemented?

[steuhs]

@adrcunha yes, we can close this
/close