tests: Run python tests with stdout unbuffered

Set the PYTHONUNBUFFERED environment variable during tests so that
python leaves stdout unbuffered. This is helpful when reading logs for
failures since the interleaved stdout and stderr will generally come out
in the right order. It's not perfect since tap-driver.sh does some
special redirection to the log file, but it's an improvement.

Closes: #1352
Approved by: dbnicholson

Makefile-tests.am

@@ -35,6 +35,7 @@ AM_TESTS_ENVIRONMENT += OT_TESTS_DEBUG=1 \
 	LD_LIBRARY_PATH=$$(cd $(top_builddir)/.libs && pwd)$${LD_LIBRARY_PATH:+:$${LD_LIBRARY_PATH}} \
 	PATH=$$(cd $(top_builddir)/tests && pwd):$${PATH} \
 	OSTREE_FEATURES="$(OSTREE_FEATURES)" \
+	PYTHONUNBUFFERED=1 \
 	$(NULL)
 if BUILDOPT_ASAN
 AM_TESTS_ENVIRONMENT += OT_SKIP_READDIR_RAND=1 G_SLICE=always-malloc