Use testing.T.TempDir in the tests

[magnusbaeck]

We can clean up the tests a bit by starting to use testing.T.TempDir instead of having this in ~20 tests:

git-repo-go/manifest/manifest_test.go

Lines 513 to 519 in 8d46bab

	tmpdir, err := ioutil.TempDir("", "git-repo")
	if err != nil {
	log.Fatal(err)
	}
	defer func(dir string) {
	os.RemoveAll(dir)
	}(tmpdir)

I would normally just have sent a PR, but testing.T.TempDir was introduced in Go 1.15 and I don't know whether it would be okay to require that compiler version given that go.mod states 1.12 and that .github/workflows/go.yml uses Go 1.14 on Mac OS X. Is there any particular reason Mac OS X builds use an older version than what's tested on Linux?

[jiangxin]

Thanks @magnusbaeck , we can upgrade Golang to 1.15 or above. Please send a PR.