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.