Refactor xunit test case builder to not use as much str addition

String concatenation in python is slow.  Refactor to not concatenate the
possibly large strings of test output and instead write them directly
to the output file.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@332064 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed