Allow 'make check PARALLEL=N' to be used to specify that N threads
are used for the python tests.  Use the default number, 5, if N is
not a number greater than 1, thus the behaviour of PARALLEL=1 and
PARALLEL=yes is unchanged.

* Makefile.in:
  (check): Pass PARALLEL value to run_tests.py

* build/run_tests.py
  (run_py_test): Handle --parallel value.
  (main): Accept --parallel value.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1573320 13f79535-47bb-0310-9956-ffa450edef68