Show pytest skip reason in CI testing / fail loud if binary does not exist

A quiet bug happened in a PR, https://github.com/pwndbg/pwndbg/actions/runs/33135781179/job/98735323269. One of the tests, test_nextret_loop was showing as "skipped", so it didn't show up as an CI error. This was running fine locally, as the compiled binary still existed because it had been compiled before, so the test found it on disk. But the source file was accidentally in a different branch, so it didn't appear in the PR and CI couldn't compile it.

On CI, it just skipped the test because the compiled binary couldn't be found at runtime. This skip happens here:

https://github.com/pwndbg/pwndbg/blob/d6d972c4f5f0c49e4bd627764ba60becc55c4d9e/tests/host/gdb/pytests_launcher.py#L47-L49

In these cases:

• The test should fail loudly
• Or, the skip reason should be printed in CI output (it just says "SKIPPED" in CI)