Eliminate boilerplate of having each test manually assign to `mydir` by calling
`compute_mydir` in lldbtest.py.
Differential Revision: https://reviews.llvm.org/D128077
This patch is similar in spirit to https://reviews.llvm.org/D84480,
but does the maccatalyst/macosx disambiguation. I also took the
opportunity to factor out the gdb-remote packet log scanning used by
several testcases into lldbutil functions.
rdar://problem/66059257
Differential Revision: https://reviews.llvm.org/D84576
