testsuite: check for FlexibleContexts in T17563

The purpose of testsuite/tests/typecheck/should_fail/T17563.hs is to
make sure we do validity checking on quantified constraints.

In particular, see the following functions in GHC.Tc.Validity:
  * check_quant_pred
  * check_pred_help
  * check_class_pred

The original bug report used a~b constraints as an example of a
constraint that requires validity checking. But with GHC Proposal #371,
equality constraints no longer require GADTs or TypeFamilies; instead,
they require TypeOperators, which are checked earlier in the pipeline,
in the renamer.

Rather than simply remove this test, we change the example to use
another extension: FlexibleContexts. Since we decide whether a
constraint requires this extension in check_class_pred, the regression
test continues to exercise the relevant code path.

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