installed tests: Allow tests to set environment variables

It's necessary sometimes for installed tests to be able to run with a
custom environment. For example, the gsocketclient-slow test requires an
LD_PRELOADed library to provide a slow connect() (this is to be added in
a followup commit).

Introduce a variable `@env@` into the installed test template, which we
can override as necessary when generating `.test` files, to run tests
prefixed with `/usr/bin/env <LIST OF VARIABLES>`.

As the only test that requires this currently lives in `gio/tests/`, we
are only hooking this up for that directory right now. If other tests in
future require this treatment, then the support can be extended at that
point.

tests/refcount/meson.build

@@ -36,6 +36,7 @@ foreach test_name, extra_args : refcount_tests
     test_conf = configuration_data()
     test_conf.set('installed_tests_dir', installed_tests_execdir)
     test_conf.set('program', test_name)
+    test_conf.set('env', '')
     configure_file(
       input: installed_tests_template,
       output: test_name + '.test',