Fix use of filenamemangle with single and multiple upstream tarballs

This patchset:

1. Tests the use of filenamemangle with single and multiple upstream tarballs. All tests use two filenamemangle patterns to represent real-world usage; e.g. in systemd and FreeType.
2. Modifies helperWatch() to accept filenamemangled filenames in test_uscan_mangle.
3. Reassigns $newfile_base to be the filename component of an upstream filenamemangled URL if the regex pattern only modifies the upstream filename.
4. Allows filenamemangling with multiple upstream tarballs in test_uscan_mangle.