Improve unit tests

[gaben]

This is also a task that I have wanted to do for a long time.

There are no functional changes; it's just a case of fixing the existing ones. The changes include:

• Correct argument order so that the correct message appears when the test errors out. For example, see ​https://github.com/JOSM/josm/blob/bdc651ba9185db66d751a9f81922a2a89ac486dc/test/unit/org/openstreetmap/josm/gui/layer/GpxLayerTest.java#L97
• Use better asserts. For example, see ​https://github.com/JOSM/josm/blob/bdc651ba9185db66d751a9f81922a2a89ac486dc/test/unit/org/openstreetmap/josm/actions/mapmode/ParallelWaysTest.java#L94
• Fix obvious typos. For example, see ​https://github.com/JOSM/josm/blob/bdc651ba9185db66d751a9f81922a2a89ac486dc/test/unit/org/openstreetmap/josm/data/validation/tests/CrossingWaysTest.java#L177
• Fix duplicates. For example, see ​https://github.com/JOSM/josm/blob/bdc651ba9185db66d751a9f81922a2a89ac486dc/test/unit/org/openstreetmap/josm/data/coor/conversion/LatLonParserTest.java#L35-L36

The first group is a JUnit 4 legacy; see ​https://docs.junit.org/6.1.3/migrating-from-junit4.html#failure-message-arguments.

I already have a patch, the examples are just teasers :)