Spike: Decide how to handle deprecated expectWarning/expectError/expectDeprecation in unit tests

[GuySartorelli]

PHPUnit 9 deprecated expecting PHP errors in unit tests. These expect methods are removed in PHPUnit 10. There is no migration path.
See also discussion in silverstripe/silverstripe-framework#10749 (comment)

• In some cases we may need to tell PHPUnit to expect some error or warning in order to test the behaviour that happens when such a warning/error is emitted (or else the test will fail, since PHPUnit converts those into exceptions).
• Some of these warnings/errors may be important to explicitly test (e.g. deprecation warnings).

We need to decide how we will handle this scenario in a consistent and systematic way.

[emteknetnz]

Based on the linked comment on the phpunit repo it seems like there's really only 2 options:
a) Remove tests for deprecations, notices etc
b) In the unit test, modify the error handler so that E_USER_* are upgraded to throwing exceptions sebastianbergmann/phpunit#5062 (comment)

We'll definitely need to do b) for the Deprecation class

I suspect we'll just make a streamlined way to do b) and upgrade all the existing expect*() calls we make. We'll could add a static withSomething(callable $func) style method to SapphireTest that automatically modifies the error_handler then resets it. Note this might break if something within $func itself modifies the error_handler