TF-A Test Framework: Rename `ptrauth` to `pauth`

This patch renames folder and source file names according to
match with TF-A naming:
- `ptrauth` folder in `tftf\tests\extensions' renamed to `pauth`,
- `test_ptrauth.c` source file to `test_pauth.c`

Signed-off-by: Alexei Fedorov <Alexei.Fedorov@arm.com>
Change-Id: Ida47e175e158e77e33270ee93ba6b3104eb0b5db