internal/coverage/cfile: snapshot test is brittle w/ respect to -coverpkg #67951
Description
At tip, if you run the internal/coverage/cfile tests with -coverpkg set to something that does include cfile, you get:
$ cd `go env GOROOT`
$ cd src/internal/coverage
$ go test -coverpkg=internal/coverage/decodecounter ./...
...
--- FAIL: TestCoverageSnapshot (0.00s)
ts_test.go:102: 0.276074 0.276074
ts_test.go:104: erroneous snapshots, C1 >= C2 = true C1=0.276074 C2=0.276074
This is happening because the test is calling out to a function that it expects will bump up the coverage percentage, but that func is not being instrumented.
The test needs to be rewritten to make it a bit more robust.