Add some OGIP files for SED / LC estimation tests (joint-crab OGIP outdated)

[cosimoNigro]

I wanted to make a quick snippet to share with the devs to illustrate a possible problem I found with LC estimation.

I realised that the only OGIP files that we have here are those in the joint-crab/spectra subdirectory.
These work fine for spectra estimation, but don't work with the LightCurveEstimator as they don't have a GTI HDU, as other OGIP files produced with more recent versions of Gammapy.

Should we add newer versions of OGIP files for these types of tests?