test/test_uscan_git: Correctly clean up testGitSignedTagWithDestDir()'s destination directory

testGitSignedTagWithDestDir() currently creates $destdir from the old $TEMP_PKG_DIR path. However, $TEMP_PKG_DIR is updated by makeDebianDir() shortly after, meaning $destdir is not cleaned up when the test ends.

This patchset fixes the order of operations to ensure $destdir is cleaned up correctly.

This patchset was originally part of !480 (merged) but was separated.