tests: Always create the scratch directory

Otherwise, some tests that try to check for flag support like

    invoke_bfs scratch -quit -xattr || skip

can be accidentally skipped if ./scratch doesn't exist yet:

    $ ./tests/tests.sh bsd/xattr
    tests skipped: 1

After this patch, we get the expected

    $ ./tests/tests.sh bsd/xattr
    tests passed: 1

1 files changed, 2 insertions, 0 deletions

diff --git a/tests/tests.sh b/tests/tests.sh
index f233f74..61e4aba 100755
--- a/tests/tests.sh
+++ b/tests/tests.sh
@@ -437,6 +437,8 @@ function make_rainbow() {
 }
 make_rainbow "$TMP/rainbow"
 
+mkdir "$TMP/scratch"
+
 # Close stdin so bfs doesn't think we're interactive
 exec </dev/null