Update unit tests from JUnit 4 to JUnit 5

[averbraeck]

JUnit 5 has been the new standard for unit tests for quite a while now (since 2017). The big challenge for updating from version 4 to version 5 is that the order of the parameters in the various static assert statements such as assertEquals or assertNull has changed. Optional arguments such as the String message were at the front, where optional arguments are typically at the back. JUnit 5 puts these optional arguments at the back, leading to a massive number of changes in the unit test code. The package structure has also changed considerably.

JUnit 5 can be included as:

<dependency>
    <groupId>org.junit.jupiter</groupId>
    <artifactId>junit-jupiter-engine</artifactId>
    <version>5.10.0</version>
    <scope>test</scope>
</dependency>

[averbraeck]

For @Rule TemporaryFolder, a replacement has to be implemented. See https://stackoverflow.com/questions/56613895/how-to-get-rid-of-temporaryfolder-rule-in-junit5 and https://www.baeldung.com/junit-5-temporary-directory

The following code:

    @Rule
    public TemporaryFolder folder= new TemporaryFolder();

can be replaced with:

    @TempDir
    private Path folder;

and a statement like:

   String temp = this.folder.getRoot().getAbsolutePath();

with

    String temp= this.folder.toString();

[averbraeck]

Upgrade executed. All unit tests pass again.