meson: Make default directory for test sockets follow session

This makes it a bit easier to override both temporary directories used
for sockets to the same place. If a directory is suitable for production
use for the session bus' temporary sockets, the same directory is very
likely to be suitable for build-time tests as well, and using that
directory for both purposes makes the tests more realistic.

The non-default CMake build system already did the equivalent of this:
it doesn't have an equivalent of test_socket_dir, and setting its
equivalent of session_socket_dir affects both.

Signed-off-by: Simon McVittie <smcv@collabora.com>

meson.build

@@ -1090,7 +1090,7 @@ if platform_unix
     test_socket_dir = get_option('test_socket_dir')
 
     if test_socket_dir == ''
-        test_socket_dir = default_socket_dir
+        test_socket_dir = session_socket_dir
     endif
 
     config.set_quoted('DBUS_TEST_SOCKET_DIR', test_socket_dir)