Eliminate boilerplate of having each test manually assign to `mydir` by calling
`compute_mydir` in lldbtest.py.
Differential Revision: https://reviews.llvm.org/D128077
The old code incorrectly calculated the start position for the search
for the third (and subsequent) instance of a particular substitution
pattern (e.g. %1).
I also added a few test cases for this parsing covering this failure.