test: Use meson add_test_setup() instead of environment variables

We add a default test setup that excludes the integration-tests suite
so that the integration tests don't run by default. This allows us to
get rid of $SYSTEMD_INTEGRATION_TESTS. Then, we add two extra setups:
'integration' and 'shell'. The 'integration' setup does not exclude the
integration-tests suite, and so can be used to run the integration tests.
The 'shell' setup does the same, but additionally sets $TEST_SHELL=1,
allowing to get rid of $TEST_SHELL in the docs.

.github/workflows/coverage.yml

@@ -90,7 +90,6 @@ jobs:
           sudo mkosi sandbox -- \
             meson setup \
             --buildtype=debugoptimized \
-            -Dintegration-tests=true \
             build
 
       - name: Build image
@@ -120,7 +119,8 @@ jobs:
             meson test \
             -C build \
             --no-rebuild \
-            --suite integration-tests \
+            --setup=integration \
+            --suite=integration-tests \
             --print-errorlogs \
             --no-stdsplit \
             --num-processes "$(($(nproc) - 1))" \

.github/workflows/mkosi.yml

@@ -197,7 +197,6 @@ jobs:
           sudo mkosi sandbox -- \
             meson setup \
             --buildtype=debugoptimized \
-            -Dintegration-tests=true \
             -Dbpf-framework=disabled \
             build
 
@@ -233,7 +232,8 @@ jobs:
             meson test \
             -C build \
             --no-rebuild \
-            --suite integration-tests \
+            --setup=integration \
+            --suite=integration-tests \
             --print-errorlogs \
             --no-stdsplit \
             --num-processes "$(($(nproc) - 1))" \