Consider separate artifacts for JUnit 4 and JUnit 5

[alvarezguille]

Now that Java UI test project is in progress TestBench has dependencies to both JUnit 4 and JUnit 5

testbench/vaadin-testbench-core/pom.xml

Lines 325 to 348 in ebb023e

	<dependency>
	<groupId>junit</groupId>
	<artifactId>junit</artifactId>
	<version>4.13.2</version>
	</dependency>
	<dependency>
	<groupId>org.junit.jupiter</groupId>
	<artifactId>junit-jupiter-api</artifactId>
	<version>${junit5.version}</version>
	<scope>provided</scope>
	</dependency>
	<dependency>
	<groupId>org.junit.jupiter</groupId>
	<artifactId>junit-jupiter</artifactId>
	<version>${junit5.version}</version>
	<scope>test</scope>
	</dependency>
	<dependency>
	<groupId>org.junit.vintage</groupId>
	<artifactId>junit-vintage-engine</artifactId>
	<version>${junit5.version}</version>
	<scope>test</scope>
	</dependency>

This can be inconvenient for users when writing tests as it's possible to accidentally mix JUnit versions and get unexpected results, for instance: wrong imports, wrong annotations or wrong asserts.

One option discussed previously was about having different specific artifacts for JUnit 4 and JUnit 5. There was a previous preparation for this approach from @Artur- by creating a shared module in #1318.

[MarcinVaadin]

Will be done as part of vaadin/flow#14784

[MarcinVaadin]

Changes pushed to #1550

[MarcinVaadin]

Reopenning as vaadin-testbench-unit-tests modules is using both junit4 and junit5 tests and it should also be splitted.