[lld-macho][nfc] Rename some tests

"stub" is a bit too overloaded... we were using it to refer to
TAPI files, but it's also the name for the PLT trampolines in Mach-O.
Going ahead, let's just use "TAPI" or ".tbd" to refer to TAPI stuff.

Reviewed By: #lld-macho, oontvoo

Differential Revision: https://reviews.llvm.org/D99807