ci: Do not use installed tests in debian-stable build

Setting installed_tests option enforces various test files to be
installed, this causes meson to build tools that might have not built
otherwise but that are still required for testing.

Also, disabling installed tests lead to slightly different code paths
when it comes to using test test files.

So, disable it for debian so that we can ensure that at test time we
have set all the dependencies between test programs and the used
resources (that can be libraries, external programs or modules).

.gitlab-ci.yml

@@ -137,7 +137,6 @@ debian-stable-x86_64:
             --libdir=lib
             -Dsystemtap=true
             -Ddtrace=true
-            -Dinstalled_tests=true
             _build
     - ninja -C _build
     - .gitlab-ci/run-tests.sh