Improve guide for testing

TESTS.md

@@ -1,25 +1,45 @@
-# Structure
+# Tests
 
-The Unit-Tests do not follow exactly the file structure of Powerlevel9k itself.
+## Automated Tests
 
-## Basic Tests
+The Unit-Tests do not follow exactly the file structure of Powerlevel9k itself,
+but we try to reflect the structure as much as possible. All tests are located
+under `test/`. Segment specific tests under `test/segments/` (one file per
+segment).
 
-Basic Tests belong in `test/powerlevel9k.spec` if they test basic functionality of
+### Installation
-Powerlevel9k itself. Basic functions from the `functions` directory have their
-Tests in separate files under `test/functions`.
 
-## Segment Tests
+In order to execute the tests you need to install `shunit2`, which is a
+submodule. To install the submodule, you can execute 
+`git submodule init && git submodule update`.
 
-These Tests tend to be more complex in setup than the basic tests. To avoid ending
+### Executing tests
-up in a huge single file, there is one file per segment in `test/segments`.
 
-# Manual Testing
+The tests are shell scripts on their own. So you can execute them right away.
+To execute all tests you could just execute `./test/suite.spec`.
+
+### General Test Structure
+
+The tests usually have a `setUp()` function which is executed before every
+test function. Speaking of, test functions must be prefixed with `test`. In
+the tests, you can do [different Assertions](https://github.com/kward/shunit2#-asserts).
+It is always a good idea to mock the program you want to test (just have a
+look at other tests), so that the testrunner does not have to have all
+programs installed.
+
+### Travis
+
+We use [Travis](https://travis-ci.org/) for Continuous Integration. This
+service executes our tests after every push. For now, we need to tell travis
+where to find the tests, which is what happens in the `.travis.yml` file.
+
+## Manual Testing
 
 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 can use either Docker or
 or our Vagrant.
 
-## Docker
+### Docker
 
 This is the easiest to use _if_ you have Docker already installed and running.
 
@@ -42,7 +62,7 @@ You can get Docker at <https://www.docker.com/community-edition>.
 
 **Note:** Not all frameworks work with all versions of ZSH (or the underlying OS).
 
-## Vagrant
+### Vagrant
 
 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!