covbot reports should not include _test.go files

[mattmoor]

Expected Behavior

Generally coverage reports elide test files.

Actual Behavior

Today the coverage report show foo_test.go files in addition to foo.go files. See here

[jessiezcc]

@steuhs, can you pls fix this?

[steuhs]

@jessiezcc yes, I already documented it internally and it's on my list to do

[steuhs]

fixed, verifiable on #1262