Merge pull request #7825 from SMillerDev/docs/formula_assertions

docs: reference formula assertions for tests

docs/Formula-Cookbook.md

@@ -261,7 +261,7 @@ We want tests that don't require any user input and test the basic functionality
 
 See [`cmake`](https://github.com/Homebrew/homebrew-core/blob/HEAD/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/HEAD/Formula/envv.rb):
+You can check that the output is as expected with `assert_equal` or `assert_match` on the output of the [Formula assertions](https://rubydoc.brew.sh/Homebrew/Assertions.html) such as in this example from the [envv formula](https://github.com/Homebrew/homebrew-core/blob/HEAD/Formula/envv.rb):
 
 ```ruby
 assert_equal "mylist=A:C; export mylist", shell_output("#{bin}/envv del mylist B").strip