Add some words about the Test-VMs

TESTS.md

@@ -12,3 +12,11 @@ Tests in separate files under `test/functions`.
 
 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`.
+
+# Test-VMs
+
+If unit tests are not sufficient (e.g. you have an issue with your prompt that
+occurs only in a specific ZSH framework), then you could use our Test-VMs!
+Currently there are two test VMs. `test-vm` is an Ubuntu machine with several
+pre-installed ZSH frameworks. And there is `test-bsd-vm` which is a FreeBSD!
+For how to run the machines see [here](test-vm/README.md).