[tests] Import executeCommand() from lit.

 - This is duplicating a small amount of code, but I don't want to consider
   lit's TestRunner module as part of its public API.


git-svn-id: https://llvm.org/svn/llvm-project/dragonegg/trunk@188001 91177308-0d34-0410-b5e6-96231b3b80d8