fix individual run of TestManifests

I observed that `go test ./diff` works, but `go test .diff -run
TestManifests` fails because TestManifests silently relies on
TestPrintDiffWithContext to call ansi.DisableColors().

Author: majewsky

diff/diff_test.go

@@ -169,6 +169,8 @@ func assertDiff(t *testing.T, before, after string, context int, stripTrailingCR
 }
 
 func TestManifests(t *testing.T) {
+	ansi.DisableColors(true)
+
 	specBeta := map[string]*manifest.MappingResult{
 		"default, nginx, Deployment (apps)": {