Revive regression test

[tebeka]

Currently the regression test is disabled - it was renamed to _regression_test.go to make the test suite skip it.

We need to:

• Rename the file back to regression_test.go
• Have it run on JSON files from testdata directory
• Create output XML files for every JSON file in testdata
• Make the diff work again

[tebeka]

Initiall, write a regression test that checks that the output of go2xunit on testdata/simple.json matches testdata/simple.junit.xml