Add info to docs about using the TEST_GROUP env variable (#5058)

* Put some info in the docs about running tests

* add info in docs/contributing about running all tests within a group

* format bash code snippets

---------

Co-authored-by: Navid C. Constantinou <navidcy@users.noreply.github.com>

Author: glwagner

docs/src/contributing.md

@@ -126,13 +126,20 @@ Oceananigans or by requesting something you think is missing.
 
 * You can test to make sure Oceananigans works by typing in `] test`. (This is equivalent to
   `using Pkg; Pkg.test()`.) Doing so will run all the tests (and this can take a while).
-  Alternatively, you can run only one test script file by providing its name as an environment
+  Alternatively, we can run only one test script file by providing its name as an environment
   variable. For example, to run the tests only from the `test_coriolis.jl` file we call:
 
-  ```
+  ```bash
   $ TEST_FILE=test_coriolis.jl julia --project -e"using Pkg; Pkg.test()"
   ```
 
+  We can also run all the tests within a certain test group, as they are defined in [`test/runtest.jl`](https://github.com/CliMA/Oceananigans.jl/blob/main/test/runtests.jl).
+  For example, to run all unit tests we call:
+
+  ```bash
+  $ TEST_GROUP=unit julia --project -e"using Pkg; Pkg.test()"
+  ```
+
 
 ## Pull Requests
 

test/runtests.jl

@@ -2,7 +2,10 @@ using Pkg
 
 include("dependencies_for_runtests.jl")
 
+# TEST_GROUP=unit julia --project -e 'using Pkg; Pkg.test()'
 group = get(ENV, "TEST_GROUP", "all") |> Symbol
+
+# TEST_FILE=test_coriolis.jl julia --project -e 'using Pkg; Pkg.test()'
 test_file = get(ENV, "TEST_FILE", :none) |> Symbol
 
 # if we are testing just a single file then group = :none