[testsuite] Remove trailing characters from command output.

When running the test suite on macOS with Python 3 we noticed a
difference in behavior between Python 2 and Python 3 for
seven.get_command_output. The output contained a newline with Python 3,
but not for Python 2. This resulted in an invalid SDK path passed to the
compiler.

Differential revision: https://reviews.llvm.org/D57275

llvm-svn: 352397