[swift-snapshot-tool] Add to the README a helper blurb about how to run tests against Swift Testing

Swift Testing is in the downloadable toolchains but is not in the normal library
lookup path, so one needs to specify it explicitly with a -I command so it is
found by the test command.

Author: gottesmm

utils/swift_snapshot_tool/README.md

@@ -101,3 +101,15 @@ Options:
 - branch: This controls the specific branch of snapshots that are downloaded. By
   default uses --development. Also supports the options --release_5_0,
   --release_6_0, --release_6_2.
+
+# FAQ and Helpful Tips
+
+## Bisecting against examples using Swift Testing
+
+Swift Testing is not stored in the normal place in a toolchain. To test against
+such an example that uses swift testing pass into ones script the following -I
+command:
+
+```
+xcrun ${SWIFTC}  "${@:1}" -o binary -parse-as-library -I ${SWIFT_LIBRARY_PATH}/testing
+```