Add streak mask plane to test exposure util

Need have a STREAK mask plane in the exposure, otherwise
the pixelFlags plugin will fail, and we cannot run
maskStreaksTask directly since it's in pipe_tasks.

python/lsst/ip/diffim/utils.py

@@ -1265,6 +1265,7 @@ def detectTestSources(exposure):
         sigma=None,  # The appropriate sigma is calculated from the PSF
         doSmooth=True
     )
+    exposure.mask.addMaskPlane("STREAK")  # Cannot run maskStreaksTask here, as it's in pipe_tasks
     selectSources = detRet.sources
     selectMeasurement.run(measCat=selectSources, exposure=exposure)