t3700-add: create subdirectory gently

The subdirectory 'sub' is created early in the test file. Later, a test case removes it during its clean-up actions. However, this test case is protected by POSIXPERM. Consequently, 'sub' remains when the POSIXPERM prerequisite is not satisfied. Later, a recently introduced test case creates 'sub' again. Use -p with mkdir so that it does not fail if 'sub' already exists. Signed-off-by: Johannes Sixt <j6t@kdbg.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>