oeqa/selftest/debuginfod: don't re-use the database

debuginfod writes the files it scans to a database in $HOME, which isn't
ideal when the build trees that get scanned typically are deleted after
the test has finished. This can result in debuginfod trying to return
objects that no longer exist on disk:

libc error: stat /home/pokybuild/yocto-worker/oe-selftest-debian/build/build-st-1032306/tmp/deploy/rpm/core2_64/elfutils-dbg-0.187-r0.core2_64.rpm: No such file or directory
libc error: stat /home/pokybuild/yocto-worker/oe-selftest-debian/build/build-st-1113320/tmp/deploy/rpm/core2_64/elfutils-dbg-0.187-r0.core2_64.rpm: No such file or directory
libc error: stat /home/pokybuild/yocto-worker/oe-selftest-debian/build/build-st-1113320/tmp/deploy/rpm/core2_64/elfutils-dbg-0.187-r0.core2_64.rpm: No such file or directory

Solve this, and save writing a database on disk at all, by using the
special database path :memory: which keeps the database in memory only,
so state can't leak between tests.

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