Make test harness more flexible with functions

Replace the 4 arguments to cmdTestCase with a function that
can let tests run arbitrary logic if it is more complex than
what is provided by the test harness. Move the existing logic
into functions that the test can use for common assertions on
golden files and golden values.

These changes were pulled out of PR #1696 to make a smaller review.

Signed-off-by: Allen Porter <allen@thebends.org>

cmd/flux/kustomization_test.go

@@ -49,7 +49,7 @@ func TestKustomizationFromGit(t *testing.T) {
 	for _, tc := range cases {
 		cmd := cmdTestCase{
 			args:            tc.args + " -n=" + namespace,
-			goldenFile:      tc.goldenFile,
+			assert:          assertGoldenFile(tc.goldenFile),
 			testClusterMode: ExistingClusterMode,
 		}
 		cmd.runTestCmd(t)