t9300: create marks files for double-import-marks test

Our tests confirm that providing two "import-marks" options in a
fast-import stream is an error. However, the invoked command would fail
even without covering this case, because the marks files themselves do
not actually exist.  Let's create the files to make sure we fail for the
right reason (we actually do, because the option parsing happens before
we open anything, but this future-proofs our test).

Signed-off-by: Jeff King <peff@peff.net>

t/t9300-fast-import.sh

@@ -2107,6 +2107,8 @@ test_expect_success 'R: abort on receiving feature after data command' '
 '
 
 test_expect_success 'R: only one import-marks feature allowed per stream' '
+	>git.marks &&
+	>git2.marks &&
 	cat >input <<-EOF &&
 	feature import-marks=git.marks
 	feature import-marks=git2.marks