build: plugins: ensure test plugins not installed in stable version

Menu Filters>Development>Demos not appear in a stable version,
and not appear in unstable version unless buildtype==debug.

plug-ins/python/meson.build

@@ -11,7 +11,7 @@ plugins = [
   { 'name': 'spyro-plus' },
 ]
 
-if not stable or not release
+if (not stable or not release) and get_option('buildtype')=='debug'
   plugins += [
     { 'name': 'test-dialog' },
   ]

plug-ins/script-fu/scripts/meson.build

@@ -1,7 +1,5 @@
 subdir('images')
-if not stable or not release
-  subdir('test')
-endif
+subdir('test')
 
 # scripts interpreted by extension-script-fu, installed to /scripts
 scripts = [

plug-ins/script-fu/scripts/test/meson.build

@@ -1,11 +1,19 @@
 # test plugins
 
-# Some, but not all, plugins in this directory are installed in unstable build.
-# None should be installed in a release.
+# All are not for ordinary users.
 # None should be translated: not marked or marked but in potfiles.skip.
 
-# Note this meson.build is excluded from a release build at a higher level,
-# so here there are no "if not release statements"
+# Some can be installed by a build, other must be manually installed.
+
+# No plugin in this directory is installed in a "stable" or "release" version,
+# even if buildtype==debug.
+
+if (stable or release)
+  subdir_done()
+elif get_option('buildtype') != 'debug'
+  subdir_done()
+endif
+
 
 # scripts interpreted by extension-script-fu, installed to /scripts
 scripts = [