Fixed .srctree tests execution

The patch fixes the following problems:
* .srctree tests pass unexpectedly on Windows, because only the first test command is executed (typically ``setup.py build_ext``).
* .srctree test commands are executed multiple times on Linux.

The origin of this bug is clearly a typo.
Linux version of ``Popen`` masked the problem by accepting newline-separated list of commands as a valid input.