x/review/git-codereview: skip tests that require "gofmt" binary when it's not available

[bcmills]

https://build.golang.org/log/36fc6a856500bd206f0f994b1565fb6badbe7034

aix-ppc64 at 6fbdfe48041c883a9f6d8c981a7205b7c327862a building review at f51a73253c4da005cfdf18a036e11185c04c8ce3
…
--- FAIL: TestGofmt (0.55s)
    gofmt_test.go:46: git-codereview gofmt -l
    util_test.go:323: died
        stdout:
        stderr:
        git-codereview: invoking gofmt: exec: "gofmt": executable file not found in $PATH
…

I'm not sure whether this should be resolved by adding gofmt to the builder's PATH, by updating the git-codereview tool to invoke go fmt instead, by updating the test to install that tool if not present, or by updating the test to skip those cases if the gofmt binary is not found.

This is technically a release-blocker via #11811, but seems pretty minor.
(CC @dmitshur @cagedmantis @toothrot @trex58)

[tklauser]

CC @Helflym

[dmitshur]

Is the problem that only the gofmt binary isn't present in PATH, or is the go binary not present on aix-ppc64 either?

The answer, from reading the failure log, is that "go" isn't available either:

exec: "gofmt": executable file not found in $PATH
...
exec: "go": executable file not found in $PATH

Tests that require the go binary usually skip themselves if the go binary isn't available (for example, see internal/testenv). So it seems reasonable to do the same for a test that requires the gofmt binary:

if !hasGofmt() {
	t.Skipf("skipping test: 'gofmt' not available on %s/%s", runtime.GOOS, runtime.GOARCH)
}

It's already being done for the git binary, see git-codereview/util_test.go.

/cc @josharian @kevinburke per owners.

If the builder can be improved to make the go, gofmt and/or git binaries available, that would be a good change to make, as it would increase the amount of tests that it is able to run. But this is a separate enhancement.

[Helflym]

gofmt is available on aix/ppc64. But for an unknown reason it was missing from the default path of the aix builder, likewise for go binary.
I've added them so let see if it's alright now.

[ianlancetaylor]

Moving to backlog milestone.