tests/.gitignore: Add test-bdrv-drain

Commit 881cfd17 added a new test binary, include it in .gitignore.

Reported-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

tests/.gitignore

@@ -18,6 +18,7 @@ test-aio
 test-aio-multithread
 test-arm-mptimer
 test-base64
+test-bdrv-drain
 test-bitops
 test-bitcnt
 test-blockjob