tests/functional: Allow asset downloading with concurrent threads

When running "make -j$(nproc) check-functional", tests that use the
same asset might be running in parallel. Improve the downloading to
detect this situation and wait for the other thread to finish the
download.

Message-ID: <20240830133841.142644-17-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
1 file changed