doc: update testing documentation

test/common/README.md

@@ -1,6 +1,29 @@
 # Node.js Core Test Common Modules
 
 This directory contains modules used to test the Node.js implementation.
+All tests must begin by requiring the `common` module:
+
+```js
+require('./common');
+```
+
+This is not just a convenience for exporting helper functions etc; it also performs
+several other tasks:
+
+* Verifies that no unintended globals have been leaked to ensure that tests
+  don't accidentally pollute the global namespace.
+
+* Some tests assume a default umask of `0o022`. To enforce this assumption,
+  the common module sets the unmask at startup. Tests that require a
+  different umask can override this setting after loading the module.
+
+* Some tests specify runtime flags (example, `--expose-internals`) via a
+  comment at the top of the file: `// Flags: --expose-internals`.
+  If the test is run without those flags, the common module automatically
+  spawns a child process with proper flags. This ensures that the tests
+  always run under the expected conditions. Because of this behaviour, the
+  common module must be loaded first so that any code below it is not
+  executed until the process has been re-spawned with the correct flags.
 
 ## Table of contents