src/sage/rings/function_field/meson.build: add function_field_test.py

I'm not sure if we should be installing pytest source files, but for
now let's do so for consistency.

Author: orlitzky

src/sage/rings/function_field/meson.build

@@ -12,6 +12,7 @@ py.install_sources(
   'function_field.py',
   'function_field_polymod.py',
   'function_field_rational.py',
+  'function_field_test.py',
   'ideal.py',
   'ideal_polymod.py',
   'ideal_rational.py',