[dotest] Avoid the need for LEVEL= makefile boilerplate

Summary:
Instead of each test case knowing its depth relative to the test root,
we can just have dotest add the folder containing Makefile.rules to the
include path.

This was motivated by r370616, though I have been wanting to do this
ever since we moved to building tests out-of-tree.

The only manually modified files in this patch are lldbinline.py and
plugins/builder_base.py. The rest of the patch has been produced by this
shell command:
  find . \( -name Makefile -o -name '*.mk' \)  -exec sed --in-place -e '/LEVEL *:\?=/d' -e '1,2{/^$/d}' -e 's,\$(LEVEL)/,,' {} +

Reviewers: teemperor, aprantl, espindola, jfb

Subscribers: emaste, javed.absar, arichardson, christof, arphaman, lldb-commits

Differential Revision: https://reviews.llvm.org/D67083

llvm-svn: 370845