tests: create XDG_RUNTIME_DIR with mode 0700

The XDG specification requires this:
    $XDG_RUNTIME_DIR defines the base directory relative to which...
    ... Its Unix access mode MUST be 0700.
    https://specifications.freedesktop.org/basedir-spec/latest/ar01s03.html

Confusingly,
    mkdir -m X -p /tmp/tarsnap-gui-test/test-blah
only applies the `-m X` to test-blah/, not tarsnap-gui-test/.  I don't
understand why POSIX specifies it as such, but at least it's clearly
defined as such in:
    https://pubs.opengroup.org/onlinepubs/9699919799/utilities/mkdir.html

tests/tests-include.pri

@@ -35,7 +35,8 @@ OBJECTS_DIR = $$TOPDIR/build/tests/
 TEST_HOME = /tmp/tarsnap-gui-test
 DEFINES += "TEST_DIR=\\\"$${TEST_HOME}/$${TARGET}\\\""
 DEFINES += "TEST_NAME=\\\"$${TARGET}\\\""
-test_home_prep.commands = rm -rf "$${TEST_HOME}/$${TARGET}"
+test_home_prep.commands = rm -rf "$${TEST_HOME}/$${TARGET}";		\
+	mkdir -m 0700 -p "$${TEST_HOME}"
 # XDG_*_HOME uses a custom "home" directory to store data, thereby not messing
 # up user-wide files.
 TEST_ENV =	XDG_CONFIG_HOME=$${TEST_HOME}				\

tests/valgrind/generate.sh

@@ -158,7 +158,7 @@ if [ "$DEBUG" -eq 1 ]; then
 fi
 
 # Create XDG dir (if necessary)
-mkdir -p "${XDGD}"
+mkdir -m 0700 -p "${XDGD}"
 
 # If the suppressions already exist, indicate that this is a retest.
 # (This may be useful while investigating occasional memory leaks.)