Remove semantics_tester cross-import from segmented_button_test.dart (#185250)

Removes the cross-import of `semantics_tester.dart` from
`material/segmented_button_test.dart`.

Duplicated `semantics_tester.dart` locally into the material test folder
to remove the dependency on widgets test utilities. This is part of the
effort to decouple Material and Cupertino from the widgets library.

Fixes part of #182636

Co-authored-by: Victor Sanni <victorsanniay@gmail.com>
Co-authored-by: Justin McCandless <jmccandless@google.com>