Testsuite: add regression test for missing class constraint

The following program is accepted by ghc-7.0 to ghc-7.10, but rejected
by ghc-6.12.3 and HEAD (and rightfully so):

    class Class1 a
    class Class1 a => Class2 a
    class Class2 a => Class3 a
    instance Class3 a => Class2 a

The last line is missing a `Class1 a` constraint. Add a regression test
for this (typechecker/should_fail/tcfail223).

Add similar missing class constraints to T7126 and T5751. I verified
that the these changes don't interfer with the intention of the tests
(they still result in a loop with ghc-7.4.1).

Reviewers: austin, simonpj, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D1078

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