Fix tests when running on Debian with debugging asserts (#1473)
A few tests failed under Debian, and a few others failed when building with NDEBUG (such that debug asserts run).
Fixes:
MultiFile would, via STLIO, call the indexing operator on an empty std::vector to get the address of the vector buffer ( &vec[0] ). This works (when debug asserts are off) but is officially not allowed on an empty vector; changed to vec.data(), which is allowed.
casacore_floatcheck, one of the python testing helper scripts, has a shebang line calling python instead of python3. This doesn't work on Debian.
One of the Dysco tests to see if Dysco doesn't crash when writing past the end of the table. In debug mode, this causes somewhere in casa an assert to fail in a check for when writing past the end of file, so this test does something which it shouldn't do.
After these changes, all tests succeed both with or without debug asserts.
