docs: contributing: adapt testing description to meson

Signed-off-by: Enrico Joerns <[email protected]>

docs/contributing.rst

@@ -85,14 +85,18 @@ Test Framework <https://developer.gnome.org/glib/stable/glib-Testing.html>`_.
 All tests reside in the ``test/`` folder and are named according to the module
 they test (``test/bundle.c`` contains tests for ``src/bundle.c``).
 
-To build and run an individual test, do::
+The tests are built by default. To explicitly switch them on or off in meson,
+use ``-Dtest=`` option::
+
+  meson setup build -Dtests=true
+
+You can run each compiled test individually::
 
-  make test/bundle.test
   ./test/bundle.test
 
 To run all tests, run::
 
-  make check
+  meson test -C build
 
 This will also run the sharness CLI tests mentioned above.