[clang-tools-extra] [llvm] Full path names are used in several unittests instead of the binary name. Fix up the testcase failures (PR #107974)

[Matthias Braun via cfe-commits]

MatzeB wrote:

Please provide additional context in your summary on how this happens.

My understanding is that this is for a setup with linux binfmt-misc + qemu running aarch64 binaries on an x86 system. The fact that we end up with absolute pathnames in `argv[0]` may be a bug in qemu or binfmt-misc or maybe it's not specified in posix etc. and any behavior is allowed (I don't really know).

Given this only affects 4 tests in all of `llvm-project` I think it is fine to just adapt the tests (even if it would turn out incorrect behavior).

So LGTM but maybe wait a couple days in case there are different opinions.

https://github.com/llvm/llvm-project/pull/107974