Use a single environment variable for everything:
- select the ATContext implementation
- select the test ATContext
- disable ATContext entirely
We use the same pattern as GSK_RENDERER, GTK_DEBUG, etc.
The documentation needs to be updated to include the environment
variable.
The installed-tests are now namespaced as gtk-4.0 to avoid colliding
with GTK+ 3, but these files weren't updated.
Signed-off-by: Simon McVittie <smcv@debian.org>