Merge pull request #10000 from palimondo/document-added-lit-substitution

Author: swift-ci

docs/Testing.rst

@@ -385,6 +385,9 @@ Other substitutions:
 * ``%platform-sdk-overlay-dir``: absolute path of the directory where the SDK
   overlay module files for the target platform are stored.
 
+* ``%swift_src_root``: absolute path of the directory where the Swift source
+  code is stored.
+
 * ``%{python}``: run the same Python interpreter that's being used to run the
   current ``lit`` test.