Add if/then/else test cases for issue #767

[Shristibot]

This PR adds additional tests for correct handling of the false schema within
conditional keywords (if, then, else), as described in Issue #767.

What these tests cover

• When if matches, then: false must cause validation to fail.
• When if does not match, else: false must cause validation to fail.
• When both then and else are false, all data must be invalid regardless
  of the branch taken.
• Ensures that false is treated as a schema that always fails, matching JSON
  Schema semantics, rather than being interpreted as {}.

These tests fill gaps in draft2020-12 conditional logic coverage, addressing the
behavior described in Issue #767.

Fixes #767.

[karenetheridge]

One test case is wrong.

I'm concerned that if it passed for your implementation, you have other issues you need to worry about! e.g. the properties keyword is never invalid when the data instance is not an object.

Also, these tests should be copied to all the version directories where they are relevant (if/then/else was added in draft7).