`./x.py test` silently ignores some of ui-fulldeps test and fails on one test

[mati865]

Yeah, if stage 1 stays a default, then we should probably

• error on x.py test <suite>/x.py test --stage 1 <suite> if the test suite requiring stage 2 is specified explicitly,
• ignore it with a message in "glob" runs like x.py test without arguments.

Originally posted by @petrochenkov in #75404 (comment)