tests: Restructure run-debuginfod-response-headers.sh

tests: Restructure run-debuginfod-response-headers.sh

run-debuginfod-response-headers.sh does occassionally fail because
it might scan an rpm more than once. Try to fix this by making sure
all files that debuginfod is supposed to scan are ready before the
server starts. And to explicitly wait till the first scan is ready
and done before testing 'scanned_files_total{source=".rpm archive"}'
instead of sending an kill -USR1.

Signed-off-by: Mark Wielaard <mark@klomp.org>