docs/Formula-Cookbook: add more details to tests

docs/Formula-Cookbook.md

@@ -257,7 +257,25 @@ function. The environment variable `HOME` is set to [`testpath`](https://rubydoc
 
 We want tests that don't require any user input and test the basic functionality of the application. For example `foo build-foo input.foo` is a good test and (despite their widespread use) `foo --version` and `foo --help` are bad tests. However, a bad test is better than no test at all.
 
-See [`cmake`](https://github.com/Homebrew/homebrew-core/blob/master/Formula/cmake.rb) for an example of a formula with a good test. The formula writes a basic `CMakeLists.txt` file into the test directory then calls CMake to generate Makefiles. This test checks that CMake doesn't e.g. segfault during basic operation. Another good example is [`tinyxml2`](https://github.com/Homebrew/homebrew-core/blob/master/Formula/tinyxml2.rb), which writes a small C++ source file into the test directory, compiles and links it against the tinyxml2 library and finally checks that the resulting program runs successfully.
+See [`cmake`](https://github.com/Homebrew/homebrew-core/blob/master/Formula/cmake.rb) for an example of a formula with a good test. The formula writes a basic `CMakeLists.txt` file into the test directory then calls CMake to generate Makefiles. This test checks that CMake doesn't e.g. segfault during basic operation.
+
+You can check that the output is as expected with `assert_equal` or `assert_match` on the output of shell_output such as in this example from the [envv formula](https://github.com/Homebrew/homebrew-core/blob/master/Formula/envv.rb):
+
+```ruby
+assert_equal "mylist=A:C; export mylist", shell_output("#{bin}/envv del mylist B").strip
+```
+
+You can also check that an output file was created:
+
+```ruby
+assert_predicate testpath/"output.txt", :exist?
+```
+
+Some advice for specific cases:
+* If the formula is a library, compile and run some simple code that links against it. It could be taken from upstream's documentation / source examples.
+A good example is [`tinyxml2`](https://github.com/Homebrew/homebrew-core/blob/master/Formula/tinyxml2.rb), which writes a small C++ source file into the test directory, compiles and links it against the tinyxml2 library and finally checks that the resulting program runs successfully.
+* If the formula is for a GUI program, try to find some function that runs as command-line only, like a format conversion, reading or displaying a config file, etc.
+* If the software cannot function without credentials or requires a virtual machine, docker instance, etc. to run, a test could be to try to connect with invalid credentials (or without credentials) and confirm that it fails as expected.
 
 ### Manuals