installed-tests: Install the slow-connect-preload.so library and use it

The gsocketclient-slow test needs this, otherwise connect() succeeds
immeidately and the test fails, because it is checking that cancellation
works. We weren't installing it for installed tests.

gio/tests/meson.build

@@ -140,11 +140,16 @@ if host_machine.system() != 'windows'
           'slow-connect-preload.c',
           name_prefix : '',
           dependencies: cc.find_library('dl'),
+          install_dir : installed_tests_execdir,
+          install: installed_tests_enabled,
         )
       ],
       'env' : {
         'LD_PRELOAD': '@0@/slow-connect-preload.so'.format(meson.current_build_dir())
       },
+      'installed_tests_env' : {
+        'LD_PRELOAD': '@0@/slow-connect-preload.so'.format(installed_tests_execdir),
+      },
       'suite': ['flaky'],
     },
     'gschema-compile' : {'install' : false},