[java] JUnitTestsShouldIncludeAssert: Add support for Hamcrest MatcherAssert.assertThat

[TheBoegl]

Affects PMD Version: 6.20.0
Rule: JUnitTestsShouldIncludeAssert
Description:
the MatcherAssert.assertThat is not counted as assertion rule

Code Sample demonstrating the issue:

import org.hamcrest.MatcherAssert;
import org.hamcrest.Matchers;
import org.junit.jupiter.api.Test;

class JUnitTestsShouldIncludeAssertRule {
    @Test
    void hamcrest_assert_should_pass_include_test() {
        MatcherAssert.assertThat("java/rule/bestpractices/JUnitTestsShouldIncludeAssertRule should pass for MatcherAssert.assertThat", "PMD 6.20.0", Matchers.is("PMD 6.20.0"));
    }
}

Running PMD through: Gradle 6.1.1

[adangel]

It makes sense, to enhance the rule to support hamcrest matchers as well. We currently support the JUnit asserts, aspectj soft asserts, Mockito calls.

In any case, we should update the rule description to clearly say, which assertion frameworks are recognized.