cmd/go: -benchtime flag should be cacheable

[bcmills]

Currently, cmd/go has a fixed list of flags for which test results may be cached:

go/src/cmd/go/internal/test/test.go

Lines 1330 to 1336 in 74cac8d

	case "-test.cpu",
	"-test.list",
	"-test.parallel",
	"-test.run",
	"-test.short",
	"-test.timeout",
	"-test.v":

Test flags outside of that list cause the test result not to be cached.

The -benchtime flag should be in the cacheable set, but today it is treated as non-cacheable.

If the -benchtime flag is set without -bench, then no benchmarks will be run and no benchmark results need to be re-run. If the -benchtime flag is set with -bench, then -bench suffices to invalidate the cache, and there is no need for -benchtime to do so as well.

(This issue is an offshoot of #44499.)

[gopherbot]

Change https://golang.org/cl/304689 mentions this issue: cmd/go: add -benchtime to cacheable test flags