Let a project type name its tests' extension

[bbatsov]

Both directions of the implementation/test toggle took the extension from the
file they started with. So in a mix project, toggling from lib/foo.ex offered
to create test/foo_test.ex - which mix test will not run, ExUnit tests being
scripts. :test-suffix only reaches the part of the name before the extension,
as the reporter found.

:src-extension and :test-extension let a type say otherwise, and default to
the old behaviour when it doesn't - so nothing changes for the languages where a
test carries its implementation's extension, which is most of them. Both
directions are covered, since going back from foo_test.exs needs to know the
implementation is .ex.

The bundled elixir type sets them, so the toggle round-trips
lib/foo.ex <-> test/foo_test.exs. They register and update like every other
type attribute, nil clearing them as usual - there's a spec for that, and for
the prefix form as well as the suffix one.

Fixes #1686