bpo-34945: Buffer output in test suite only when creating junit file (GH-10204)

After commit d0f49d2f5085ca68e3dc8725f1fb1c9674bfb5ed, the output of the
test suite is always buffered as the test output needs to be included in
the JUnit file in same cases (as when a test fails). This has the
consequence that printing or using debuggers (like pdb) in the test
suite does not result in a good user experience anymore.

This commit modifies the test suite runner so it only captures the test
output when the JUnit file is requested to fix the regression so prints
and debuggers are usable again.