[docs] Document the swift-lldb test whitelist (swiftlang#18730)

Author: vedantk

docs/Testing.md

@@ -501,3 +501,18 @@ code that uses an ``if true {}`` or similar no-op scope instead of
 If you're specifically testing the autoreleasing behavior of code, or do not
 expect code to interact with the Objective-C runtime, it may be OK to use ``if
 true {}``, but those assumptions should be commented in the test.
+
+#### Enabling/disabling the lldb test whitelist
+
+It's possible to enable a whitelist of swift-specific lldb tests to run during
+PR smoke testing. Note that the default set of tests which run (which includes
+tests not in the whitelist) already only includes swift-specific tests.
+
+Enabling the whitelist is an option of last-resort to unblock swift PR testing
+in the event that lldb test failures cannot be resolved in a timely way. If
+this becomes necessary, be sure to double-check that enabling the whitelist
+actually unblocks PR testing by running the smoke test build preset locally.
+
+To enable the lldb test whitelist, add `-G swiftpr` to the
+`LLDB_TEST_CATEGORIES` variable in `utils/build-script-impl`. Disable it by
+removing that option.