lib/tests: Unify documentation of individual testable files

lib/path/tests/prop.sh

···
 #!/usr/bin/env bash
 
-# Property tests for the `lib.path` library
-#
+# Property tests for lib/path/default.nix
 # It generates random path-like strings and runs the functions on
 # them, checking that the expected laws of the functions hold
+# Run:
+# [nixpkgs]$ lib/path/tests/prop.sh
+# or:
+# [nixpkgs]$ nix-build lib/tests/release.nix
 
 set -euo pipefail
 shopt -s inherit_errexit

lib/tests/misc.nix

···
-# to run these tests:
-# nix-instantiate --eval --strict nixpkgs/lib/tests/misc.nix
-# if the resulting list is empty, all tests passed
+/*
+Nix evaluation tests for various lib functions.
+
+Since these tests are implemented with Nix evaluation, error checking is limited to what `builtins.tryEval` can detect, which is `throw`'s and `abort`'s, without error messages.
+If you need to test error messages or more complex evaluations, see ./modules.sh, ./sources.sh or ./filesystem.sh as examples.
+
+To run these tests:
+
+  [nixpkgs]$ nix-instantiate --eval --strict lib/tests/misc.nix
+
+If the resulting list is empty, all tests passed.
+Alternatively, to run all `lib` tests:
+
+  [nixpkgs]$ nix-build lib/tests/release.nix
+*/
 with import ../default.nix;
 
 let

lib/tests/modules.sh

···
 #!/usr/bin/env bash
-#
+
 # This script is used to test that the module system is working as expected.
+# Executing it runs tests for `lib.modules`, `lib.options` and `lib.types`.
 # By default it test the version of nixpkgs which is defined in the NIX_PATH.
+#
+# Run:
+# [nixpkgs]$ lib/tests/modules.sh
+# or:
+# [nixpkgs]$ nix-build lib/tests/release.nix
 
 set -o errexit -o noclobber -o nounset -o pipefail
 shopt -s failglob inherit_errexit

lib/tests/sources.sh

···
 #!/usr/bin/env bash
+
+# Tests lib/sources.nix
+# Run:
+# [nixpkgs]$ lib/tests/sources.sh
+# or:
+# [nixpkgs]$ nix-build lib/tests/release.nix
+
 set -euo pipefail
 shopt -s inherit_errexit