Use .json file for tests

Created by: scarlehoff

Since now we have a .json file with all the information from the fit, I thought it made sense to use this instead of the "legacy files" in order to do a regression test.

Since the .fitinfo file in particular is still being used by validphys (and postfit), the test also ensures that the information in the .fitinfo file and the .json file is the same.

Finally, I added a second regression test since replica 1 doesn't pass the positivity veto so that we have one that passes (replica 2) (and therefore stops early) and one that doesn't. The double one is only ran in Linux, in Mac I'm running only replica 1, since the fit takes x3 in travis.