test: add SQL equality assertion helper

[memleakd]

Description

This PR adds assertSqlEquals() to CIUnitTestCase.

This follows up on @datamweb's suggestion to introduce this helper.

The helper keeps a small pattern we already use in many Query Builder tests in one place:

$this->assertSqlEquals($expected, $builder->getCompiledSelect());

It intentionally stays conservative and only ignores newlines in the actual SQL. It does not lowercase, trim, or normalize whitespace more broadly, so SQL assertions remain strict while avoiding the repeated str_replace("\n", ' ', ...) calls.

I also updated the testing docs and converted a few representative Query Builder assertions to show the intended usage. Broader migration of existing SQL assertions can happen separately after the helper is available.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[datamweb]

@memleakd Thanks! LGTM.