CTest: Support Multiple -L label filters

For #21087 (closed) :

When -L is given multiple times, then a test must satisfy each RE (with some label on the test) in order to pass the filter. With a single -L, the functionality is unchanged; with a single -L, an RE can write an "or" on labels (e.g. label1 OR label2 is simply `-L 'label1|label2'). What the RE cannot effectively do, is demand more than one label (e.g. label1 AND ALSO label2) on the test.

In this branch, accumulate -L to a collection of RE's; a test must satisfy all of the RE's in order to be run.