Fix: Python agent tests are always skipped

v2: Change the configure report section to emulate the java ust agent test
reporting.

Introduce --enable-test-python{2,3}-agent and --enable-test-python-agent-all
flag on configure.

Configure searches for the python agent for both Python 2 and 3 and
enables or skips their associated tests based on the result.

When using the --enable-test-python{2,3}-agent & --enable-test-python-agent-all
flags, a strict checks on tests dependancies is performed and fails the
configure instead of simply disabling the tests.

--disable* flags can be used to force tests skipping.

Also fixes a minor bug in agent test on enabling event with filtering.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>