test: Install files required to run integration tests standalone

Let's make it possible to run the integration tests without requiring
the sources by installing the mkosi and integration-tests directories.

meson.build

@@ -2692,6 +2692,13 @@ if mkosi.found()
         )
 endif
 
+if install_tests
+        install_subdir('mkosi',
+                       install_dir : testsdir,
+                       exclude_files : ['mkosi.local.conf', 'mkosi.key', 'mkosi.crt'],
+                       exclude_directories : ['mkosi.local'])
+endif
+
 ############################################################
 
 subdir('rules.d')