Add a new TestDialect directory in tests/. This directory defines a fake...

    Add a new TestDialect directory in tests/. This directory defines a fake 'TestDialect' that allows for the use of FileCheck to test things that aren't currently used anywhere else in tree. As a first order, this should simplify the tests used for tablegen components revolving around operation constraints/patterns.

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PiperOrigin-RevId: 249724328