Add documentation for Unit Tests

TESTS.md

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+# Structure
+
+The Unit-Tests do not follow exactly the file structure of Powerlevel9k itself.
+
+## Basic Tests
+
+Basic Tests belong in `test/powerlevel9k.spec` if they test basic functionality of
+Powerlevel9k itself. Basic functions from the `functions` directory have their
+Tests in separate files under `test/functions`.
+
+## Segment Tests
+
+These Tests tend to be more complex in setup than the basic tests. To avoid ending
+up in a huge single file, there is one file per segment in `test/segments`.