Fix some whitelisted PHPStan errors

[morozov]

Q	A
Type	improvement
BC Break	yes
Fixed issues	#3603, partially

1. Some errors are no longer relevant due to the code changes and issues fixed in PHPStan.
2. The RunSqlCommandTest is reworked to comply with its type constraints.
3. Type tests were reworked to test their respective classes w/o using the type registry (less coupling). Additionally, the platform mocks were reworked to mock the expected behavior explicitly instead of relying on the default platform logic (less coupling).
4. The absence of the primary key is indicated by Table::$_primaryKey = null instead of false (the actual BC break).
5. Removed the preg_replace() error from the whitelist and added an assertion. Unless it's known under which circumstances a NULL may be returned (as pointed out in PHPStan is too strict about preg_replace() phpstan/phpstan#1901), we cannot test it. Hence, it's an assertion, not a condition with an exception.