Remove nested test classes from practice exercise simple-cipher #2538
Description
Due to a bug in the Java test runner the current tests for the simple-cipher exercise fail to run on Exercism.
Since this bug is not easily fixed, the best workaround is to refactor the exercise tests into a single class containing all of the test methods.
Implementation notes
Since some tests have the same method name (e.g. RandomKeyCipher.cipherCanDecode() and SubstitutionCipher.cipherCanDecode()) the test methods need to be renamed.
When doing so, make sure to have a look at the exercise's .meta/tests.toml file.
This contains descriptions for each test; try to have the test method names reflect the descriptions when possible.
This makes it easy for future maintenance to determine which tests have been implemented already.
Contribution notes
As always, make sure to follow the Exercism contributing guidelines and the Java track contributing guidelines.
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