The afw package uses a file named: test.fits in multiple testers. If the user sets up the build to use multiple CPU (-j #), then there is the risk that the shared filename will be affected by more than one tester at a time.
In the case which provoked this Issue, the tester: testSimpleTable.py, reported that the file was missing. A simple rerun managed to get past the error.
I recommend that the different testers use uniquely named demo files.