[llvm] [Support] Show '[subcommand]' in the help for less than 3 subcommands (PR #74557)

[Igor Kudrin via llvm-commits]

igorkudrin wrote:

Hmm, I was under the impression that gtest runs unit tests in parallel, so it is not possible to intercept stdout. But in fact, gtest is single-threaded, and moreover, there are other tests in the test suite that already intercept the stream. So I added tests for the change to the patch.

https://github.com/llvm/llvm-project/pull/74557