[libcxx] [test] Make the condvar wait_for tests a bit more understandable. NFC.

This was requested in the review of D99175; rename the "runs"
variable to clarify what it means wrt the test, and move updating of
it to the main function to clarify its behaviour wrt the two runs
further.

Differential Revision: https://reviews.llvm.org/D99768

GitOrigin-RevId: f8013a35b6fb02ad35008c373f7b06a24b5dd8e4
2 files changed