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>
diff --git a/tests/.gitignore b/tests/.gitignore
index 74e55d7..e5c744b 100644
--- a/tests/.gitignore
+++ b/tests/.gitignore
@@ -18,6 +18,7 @@
 test-aio-multithread
 test-arm-mptimer
 test-base64
+test-bdrv-drain
 test-bitops
 test-bitcnt
 test-blockjob