tests/gsubprocess: Locate test child binary for installed tests too

Just use the new g_test_build_filename() rather than assuming
it's in cwd.
Add index for 2.40 api additions
Add some missing argument docs
Generate introspected docs for GSimpleProxyResolver
Fix up gio-sections.txt
Fix up subprocess docs

The GSubprocessLauncher docs had their own long description,
but were not properly set up as their own section.
Include gapplication(1) in the tools section