Improve PHPUnit-based tests discoverability

Problem/Motivation

Currently Drupal test discovery system is looking for Unit/Kernel/Functional/FunctionalJavascript subdirectories within MODULENAME/tests/src directory. There are no possibility for a module to create a base test class/trait and share it between Functional and FunctionalJavascript tests for example.
See \Drupal\simpletest\TestDiscovery:

// Add PHPUnit test namespaces.
$this->testNamespaces["Drupal\\Tests\\$name\\Unit\\"][] = "$base_path/tests/src/Unit";
$this->testNamespaces["Drupal\\Tests\\$name\\Kernel\\"][] = "$base_path/tests/src/Kernel";
$this->testNamespaces["Drupal\\Tests\\$name\\Functional\\"][] = "$base_path/tests/src/Functional";
$this->testNamespaces["Drupal\\Tests\\$name\\FunctionalJavascript\\"][] = "$base_path/tests/src/FunctionalJavascript";

On the other hand, Simpletest allows for any testing configuration within the module:

// Add Simpletest test namespace.
$this->testNamespaces["Drupal\\$name\\Tests\\"][] = "$base_path/src/Tests";

Proposed resolution

Allow adding any configuration of PHPUnit-based tests within modules, but advice to use aforementioned structure.

Remaining tasks

Provide a patch.

User interface changes

None

API changes

None

Data model changes

None