post/output.php tests are leaky

[boonebgorges]

The $pages global is getting populated in Tests_Post_Output::test_the_content_attribute_value_with_colon(), but not being reset properly.

See #36934.

[boonebgorges]

In 38678:

Tests: Reset post-related globals after each test.

Globals like $pages were leaking between tests, resulting in various
bits of weirdness.

Globals will kill WordPress. Globals are killing WordPress.

See #38196.

[boonebgorges]

In 38679:

Tests: Move get_the_excerpt() tests to their own file.

See #36934. Fixes #38196.