doc: document directories in test directory

test/README.md

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+# Tests
+
+### abort
+
+Tests for when the `--abort-on-uncaught-exception` flag is used.
+
+| Runs on CI |
+|:----------:|
+| No         |
+
+### addons
+
+Tests for [addon](https://nodejs.org/api/addons.html) functionality.
+
+| Runs on CI |
+|:----------:|
+| Yes        |
+
+### cctest
+
+C++ test that is run as part of the build process.
+
+| Runs on CI |
+|:----------:|
+| Yes        |
+
+### debugger
+
+Tests for [debugger](https://nodejs.org/api/debugger.html) functionality.
+
+| Runs on CI |
+|:----------:|
+| No         |
+
+### disabled
+
+Tests that have been disabled from running for various reasons.
+
+| Runs on CI |
+|:----------:|
+| No         |
+
+### fixtures
+
+Test fixtures used in various tests throughout the test suite.
+
+### gc
+
+Tests for garbage collection related functionality.
+
+| Runs on CI |
+|:----------:|
+| No         |
+
+### internet
+
+Tests that make real outbound connections (mainly networking related modules).
+Tests for networking related modules may also be present in other directories,
+but those tests do not make outbound connections.
+
+| Runs on CI |
+|:----------:|
+| No         |
+
+### known_issues
+
+Tests reproducing known issues within the system.
+
+| Runs on CI |
+|:----------:|
+| No         |
+
+### message
+
+Tests for messages that are output for various conditions (`console.log`,
+error messages etc.)
+
+| Runs on CI |
+|:----------:|
+| Yes        |
+
+### parallel
+
+Various tests that are able to be run in parallel.
+
+| Runs on CI |
+|:----------:|
+| Yes        |
+
+### pummel
+
+Various tests for various modules / system functionality operating under load.
+
+| Runs on CI |
+|:----------:|
+| No         |
+
+### sequential
+
+Various tests that are run sequentially.
+
+| Runs on CI |
+|:----------:|
+| Yes        |
+
+### testpy
+
+Test configuration utility used by various test suites.
+
+### timers
+
+Tests for [timing utilities](https://nodejs.org/api/timers.html) (`setTimeout`
+and `setInterval`).
+
+| Runs on CI |
+|:----------:|
+| No         |