tc-testing: gitignore, ignore generated test results

Ignore any .tap or .xml test result files generated by tdc.

Additionally, ignore plugin symlinks.
Signed-off-by: NLucas Bates <lucasb@mojatatu.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

tools/testing/selftests/tc-testing/.gitignore

 __pycache__/
 *.pyc
+plugins/
+*.xml
+*.tap