GODT-1523: Reduce unnecessary shell executions. Inspired by @kortschak.

- check opened file descriptors
- detection of darwin version
- detection of apple interface color theme
- test for update sync (copy files)
- replace exec with execabs

test/context/credentials.go

@@ -35,7 +35,7 @@ type fakeCredStore struct {
 
 // newFakeCredStore returns a fake credentials store (optionally configured with the given credentials).
 func newFakeCredStore(initCreds ...*credentials.Credentials) (c *fakeCredStore) {
-	c = &fakeCredStore{credentials: map[string]*credentials.Credentials{}}
+	c = &fakeCredStore{credentials: map[string]*credentials.Credentials{}, failOnWrite: false}
 	for _, creds := range initCreds {
 		if creds == nil {
 			continue