oeqa/selftest/debuginfod: improve testcase

Primarily, before running the debuginfod-find tool, check that the
debuginfod server has finished sweeping the deploy directory.  If we
make the request too soon then there's a rare chance that we run the
client before it has scanned the right packages, and the log gets
swamped with warnings from sqlite due to a race.

Also:
- unset DEBUGINFOD_URLS so the debuginfod doesn't proxy to an upstream
  server provided by the host distro
- Lower concurrency to reduce system load and handle systems with lower
  maximum open file counts but lots of cores (as the concurrency means
  cores*2*2 open files)
- Set the refresh times to 0 so we never rescan during the test
- Only scan the packages for the format which the image is using
- Log the commands that are being invoked

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>