Add missing equivalences for XX{Plus,Minus}YYGate

[jakelishman]

Summary

These are just copied from the _define method on the classes. The equivalence-library tests are updated (since subTest doesn't isolate correctly with testtools) to print out all the names of the tests being run, so it's clear that the existing test now tests these definitions.

Details and comments

Bonus points to anyone who already knew the esoteric Python scoping rules that forbade me from using a list comprehension to define the test parametrisation here.

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[coveralls]

Pull Request Test Coverage Report for Build 3345143334

• 16 of 16 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 84.492%

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Totals
Change from base Build 3343035335:	0.01%
Covered Lines:	62175
Relevant Lines:	73587

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💛 - Coveralls

[Cryoris]

LGTM 👍🏻 Would it make sense to automatically add the default decomposition of gates into the equivalence library instead of copy+pasting the actual definition? Or maybe the other way around: <SomeStandardGate>._define invokes the equivalence library and runs that (but maybe this way around it's too slow...)

[jakelishman]

I think (but am speaking purely from my own views) that the general direction we're heading in is to have the equivalence library be the primary source of definitions for gates, at least, and _define just be a user convenience for custom gates. I haven't thought enough about that yet - for now, copy-paste is probably enough.

(This gate would already have been fine in transpile I think, because it would have been unrolled before hitting the 1q optimiser that caused a test failure in a linked PR.)