Fix empty xunit file on error.

[mjonss]

Close: #116

Issue was that os.Exit(1) was used on error, which bypasses all defer function.

Fix consistently use the already existing global testSuite and write the xunit file explicitly in the end of the main function, before possible os.Exit() calls.

[CLAassistant]

All committers have signed the CLA.

[wuhuizuo]

@mjonss Could we merge it?