[libcxx][lit] Fix incorrect lambda capture in hasLocale checks

The lambda being used to check whether locales are supported was always
passing the value of alts from the last loop iteration due to the way that
python lambda captures work. Fix this by using a default argument capture.

To help debug future similar issues I also added a prefix to the config
test binary indicating which locale is being tested.
I originally found this issue when implementing a new executor that simply
collects test binaries in a given directory and was surprised to see many
additional executables other than the expected test binaries. I therefore
added the locale prefix to the test binaries and noticed that they were all
checking for cs_CZ.ISO8859-2.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D84040