mkosi: Don't run slow tests by default

Instead, allow enabling it via an environment variable and do so
in CI.

.github/workflows/mkosi.yml

@@ -86,6 +86,7 @@ jobs:
         [Content]
         Environment=CI_BUILD=1
                     DEFAULT_TIMEOUT_SEC=90
+                    SLOW_TESTS=true
 
         [Output]
         KernelCommandLineExtra=systemd.unit=mkosi-check-and-shutdown.service

mkosi.presets/00-base/mkosi.build

@@ -60,7 +60,7 @@ if [ ! -f "$BUILDDIR"/build.ninja ]; then
         -D b_sanitize="${SANITIZERS:-none}"
         -D install-tests=true
         -D tests=unsafe
-        -D slow-tests=true
+        -D slow-tests="${SLOW_TESTS:-false}"
         -D utmp=true
         -D hibernate=true
         -D ldconfig=true