Add Scala2 library TASTy testing instructions (#19218)

[skip ci]

Author: bishabosha

docs/_docs/contributing/testing.md

@@ -155,6 +155,34 @@ To run `testCompilation` on a bootstrapped Dotty compiler, use
 Some tests can only be run in bootstrapped compilers; that includes all tests
 with `with-compiler` in their name.
 
+### Scala 2 library TASTy tests
+
+Usually we use the Scala 2 library JAR (with classfiles) generated by Scala 2. We can
+also use a special version of the library that we generate with only TASTy files. This
+JAR is added to the classpath before the Scala 2 library JAR. This allows the compiler
+to load the TASTy and the runtime to load the original classfiles.
+
+The library is compiled in `scala2-library-bootstrapped` with TASTy and classfiles.
+These classfiles should not be used. The `scala2-library-tasty` project repackages the
+JAR `scala2-library-bootstrapped` to only keep TASTy files. The `scala2-library-tasty-tests` provides some basic tests using the library TASTy JAR.
+
+```
+$ sbt
+> scala2-library-tasty/compile
+> scala2-library-tasty-tests/run
+> scala2-library-tasty-tests/test
+```
+
+We can enable this library in the build using the SBT setting `useScala2LibraryTasty`. This setting can only be used by bootstrapped compiler tests and is currently only supported for `test` (or `testCompilation`) and `scalac` (or `run`).
+
+```
+$ sbt
+> set ThisBuild/Build.useScala2LibraryTasty := true
+> scala3-compiler-bootstrapped/scalac MyFile.scala
+> scala3-compiler-bootstrapped/test
+> scala3-compiler-bootstrapped/testCompilation
+```
+
 ### From TASTy tests
 
 `testCompilation` has an additional mode to run tests that compile code from a `.tasty` file.