core: introduce specifiers for /tmp and /var/tmp

This corresponds nicely with the specifiers we already pass for
/var/lib, /var/cache, /run and so on.

This is particular useful to update the test-path service files to
operate without guessable files, thus allowing multiple parallel
test-path invocations to pass without issues (the idea is to set $TMPDIR
early on in the test to some private directory, and then only use the
new %T or %V specifier to refer to it).

test/test-execute/exec-specifier.service

@@ -16,6 +16,8 @@ ExecStart=test %t = /run
 ExecStart=test %S = /var/lib
 ExecStart=test %C = /var/cache
 ExecStart=test %L = /var/log
+ExecStart=test %T = /tmp
+ExecStart=test %V = /var/tmp
 ExecStart=sh -c 'test %u = $$(id -un 0)'
 ExecStart=test %U = 0
 ExecStart=sh -c 'test %h = $$(getent passwd 0 | cut -d: -f 6)'