Migration to JUnit5

[georgweiss]

This PR is to migrate all unit tests to JUnit5. The version to use is defined in top-level pom.xml, but future updates of (for instance) Spring Boot may pull in a newer version.

New/updated code depending on some unit test library (e.g. testfx) should make sure to exclude JUnit4, i.e.

 <exclusions>
    <exclusion>
        <groupId>junit</groupId>
        <artifactId>junit</artifactId>
    </exclusion>
</exclusions>

There are some changes developers should be acquainted with. I list them here for the sake of information. It is not an exhaustive list:

• Some test cases require a newer version of the Surefire Maven plugin (3.0.0-M7). This has been updated in the top level pom.
• Assert has been replaced by Assertions.
• @Before and @After have been replaced by @BeforeAll and @AfterAll.
• Usage of assertThat has been updated to use the Hamcrest API in MatcherAssert.
• Constructs like @Test(expected = SomeExceptionClass.class) are not supported. One may use assertThrows instead.
• @RunWith(SpringRunner.class) is replaced by @ExtendWith(SpringExtension.class)
• A string message in for instance assertEquals has to be the last parameter, not first.

On top the adaptations to the JUnit5 APIs, this PR also includes a few updates to eliminate warnings.

[shroffk]

There are some places where the import statements could be expanded instead of the use of wildcards