[llvm-xray][test] Combine two AArch64 tests

Rename extract-instrmap-aarch64.test to extract-instrmap.test because
the path component `AArch64` conveys the target name clearly.

Additionally, adopt a convention we start to use in LLVM binary
utilities: prepend `#` to CHECK/RUN lines and `##` to comment lines even
if the file contains no code. The notation makes CHECK/RUN/comments
stand out.

Reviewed By: dberris

Differential Revision: https://reviews.llvm.org/D77883