Support PHPUnit 8 optionally in Drupal 8, while keeping support for ^6.5 and ^7

Problem/Motivation

Similarly to #2950132: Support PHPUnit 7 optionally in Drupal 8, while keeping support for ^6.5, this issue is about enabling testing with PHPUnit 8 in Drupal 8.

PHPUnit 6 is out of support since Feb 1, 2019
PHPUnit 7 will be end of support Feb 7, 2020
PHPUnit 8 will be supported until Feb 5, 2021
PHPUnit 9 will be released Feb 2020.

The big problem with PHPUnit 8 is that it is changing the signature of the main test methods that test classes extend - above all setUp() and tearDown.

This means that ALL the 1700+ implementations of setUp() and the 30 of tearDown() in Drupal test code will have to be touched one way or another - unless I'm mistaken.

Also, in PHPUnit8 several assertions methods are deprecated for removal in PHPUnit9, for example:

• Using assertContains() with string haystacks is deprecated and will not be supported in PHPUnit 9. Refactor your test to use assertStringContainsString() or assertStringContainsStringIgnoringCase() instead.
• The @expectedException, @expectedExceptionCode, @expectedExceptionMessage, and @expectedExceptionMessageRegExp annotations are deprecated. They will be removed in PHPUnit 9. Refactor your test to use expectException(), expectExceptionCode(), expectExceptionMessage(), or expectExceptionMessageRegExp() instead.
• assertInternalType() is deprecated and will be removed in PHPUnit 9. Refactor your test to use assertIsArray(), assertIsBool(), assertIsFloat(), assertIsInt(), assertIsNumeric(), assertIsObject(), assertIsResource(), assertIsString(), assertIsScalar(), assertIsCallable(), or assertIsIterable() instead.
• assertArraySubset() is deprecated and will be removed in PHPUnit 9.
• getObjectAttribute() is deprecated and will be removed in PHPUnit 9.
• readAttribute() is deprecated and will be removed in PHPUnit 9.

Open points

• None at the moment.

Spun off issues

• MigrateExecutableMemoryExceededTest - it has mock expectation fails in PHPUnit8 due to inconsistent matches(). Added a @todo and commented out the failing tests. #3102903: MigrateExecutableMemoryExceededTest has mismatched argument type mock expectations (and fails in PHPUnit8)
• ViewExecutableTest - it has mock expectation fails in PHPUnit8 due to callback return type should be bool and not string. Added a @todo and commented out the failing tests. #3102899: ViewExecutableTest uses a mocked argument callback wrongly (and fails in PHPUnit8)
• #3094151: ExpectDeprecationTrait is not compatible with PHPUnit 8
• #3097822: Drupal\migrate_drupal\Tests\StubTestTrait is not compatible with PHPUnit 8

Proposed resolution

I can see two options:

1. Change the signatures of the methods in Drupal test code according to PHPUnit 8

This will have the impact of forcing testing only on PHP 7.2+ (the minimum allowed by PHPUnit8), and from the day such a change will be in core, all contrib module will have to comply or fail tests.

2. Introduce a compatibility layer

1. A new Drupal\Tests\BaseTestClass, extending from PHPUnit\Framework\TestCase, is the new base class for ANY Drupal test, being Unit, Kernel, Functional, etc
2. The Drupal\Tests\BaseTestClass is actually aliased to a Drupal\TestTools\PhpUnitCompatibility\PhpUnit#\BaseTestCase class, one for each PHPUnit version, using the compatibility layer mechanism put in place to support PHPUnit 7. These aliased classes hold the override of the ::setUp(), ::tearDown(), etc methods, type hinted according to the specific PHPUnit version.
3. Each overridden method calls a corresponding proxy in the test class so ::setUp() -> ::xxxSetup(). The naming tbd.
4. ALL test classes in Drupal have to rename their PHPUnit overriding methods to the proxies, and any call to parent::method() to the parent proxy, too i.e. parent::setUp() --> parent::xxSetUp()

Remaining tasks

A lot

User interface changes

None

API changes

Some

Data model changes

None

Release notes snippet

TBD