cmd{cover,covdata,go}: better coverage for tests that build tools
Some of the unit tests in Go's "cmd" tree wind up building a separate
copy of the tool being tested, then exercise the freshly built tool as
a way of doing regression tests. The intent is to make sure that "go
test" is testing the current state of the source code, as opposed to
whatever happened to be current when "go install <tool>" was last run.
Doing things this way is unfriendly for coverage testing. If I run "go
test -cover cmd/mumble", and the cmd/mumble test harness builds a
fresh copy of mumble.exe, any runs of that new executable won't
generate coverage data.
This patch updates the test harnesses to use the unit test executable
as a stand-in for the tool itself, so that if "go test -cover" is in
effect, we get the effect of building the tool executable for coverage
as well. Doing this brings up the overall test coverage number for
cmd/cover quite dramatically:
before change:
$ go test -cover .
ok cmd/cover 1.100s coverage: 1.5% of statements
after change:
$ go test -cover .
ok cmd/cover 1.299s coverage: 84.2% of statements
Getting this to work requires a small change in the Go command as
well, to set GOCOVERDIR prior to executing a test binary.