REST API: Improve term meta tests

[SergeyBiryukov]

Bringing over @birgire's patch from #39122:

• Removes duplicated array key assertion: $this->assertArrayHasKey( 'meta', $data );
• Adds @ticket annotations.
• Use assertSame instead of assertEquals, as null, false and '' are equal under assertEquals.
• Use assertFalse instead of assertEquals( false, ... )

[danielbachhuber]

Bumping to 5.1 because it's not critical for 5.0. The patch could use a refresh too.

[pento]

In 44510:

Tests: Improve REST API tests for categories and tags.

Props birgire, SergeyBiryukov.
See #39122.
Fixes #45077.