Use Spekt for test result files

[abonie]

Recent migration to MTP made it so that test result files have more enigmatic names:

while before they were more meaningful (and also split differently):

We can try using Spekt with --report-spekt-xunit --report-spekt-xunit-filename <file-name-containing-placeholders> to get that fixed. It will require XunitXml.TestLogger 8.0.0 or newer.