Tidy up `pragma comment lib` handling and testing

A bit of historical research shows that over the years:
Commit 99efc036 added `pragma comment lib` support for PS4.
Commit fd4db533 added `pragma comment lib` support for all ELF targets.
Commit 1d16515f reworked dependent-library support for all ELF targets.

The upshot is that some PS4-specific code became dead, and the
testing became somewhat fragmented.  I've removed the dead code and
combined the previous PS4-specific and linux-specific tests for the
diagnostics into one generic ELF test.
Also added a couple of PS5 runs while I was in there.