testsuite: introspection: Do not override environment variables.

testsuite: introspection: Do not override environment variables.

Functional package managers such as GNU Guix rely on environment
variables such as GI_TYPELIB_PATH to discover the system libraries and
resources; extend rather than override them.

* testsuite/introspection/meson.build (env): New variable that extends
rather than override the GI_TYPELIB_PATH and LD_PRELOAD environment
variables.
(api): Use the above as the value of the 'env' keyword argument.