[mutter] docs: Update tests instructions

[Georges Basile Stavracas Neto <gbsneto src gnome org>]

commit a38bae259ef48dd1369a7b2a57f0419016120aad
Author: Douglas R. Reno <renodr linuxfromscratch org>
Date:   Sat Apr 27 00:12:04 2019 +0000

    docs: Update tests instructions
    
    The documentation still refers to autotools, update it to
    use the corresponding meson commands.
    
    https://gitlab.gnome.org/GNOME/mutter/issues/568

 src/tests/README | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)
---
diff --git a/src/tests/README b/src/tests/README
index 2038eb825..9c620c328 100644
--- a/src/tests/README
+++ b/src/tests/README
@@ -29,10 +29,9 @@ The tests are installed according to:
 
 https://wiki.gnome.org/Initiatives/GnomeGoals/InstalledTests
 
-if --enable-installed-tests is passed to configure. You can run them
-uninstalled with:
+if -Dtests=true is passed to `meson configure`. You can run them uninstalled with:
 
- cd src && make run-tests
+ ninja test
 
 Command reference
 =================