AST-1225 Add UVW flagging test to GainCal test

This MR adds test_uvwflagging() to the GainCal integration tests

Also the test for DDECal is updated. For GainCal it turned out that modified flags were passed on to the next step, but that these modified flags were not written to msout. The test now not only checks msout, but also adds a second calibration step, that should not see the flags of the previous step.

GainCal did indeed modify flags that referenced the original flags. This has been fixed.