[llvm] [docs] Add Included Suites to Test Suite Guide (PR #128937)

[Tarun Prabhu via llvm-commits]

================
@@ -352,6 +352,27 @@ performance. You can find more information at
 of the LLVM project is hosted at [http://lnt.llvm.org](http://lnt.llvm.org).
 
 
+Included Test Suites
+--------------------
+
+The LLVM Test Suite repository includes a number of test suites that we have
+copied into the repository from external sources, and hand-written CMake files
+to hook into the LLVM Test Suite system. For example, these include GCC's C
+and Fortran Torture test suites.
----------------
tarunprabhu wrote:

NIT: Slight change in wording to be a bit more accurate: we include all the gfortran tests, not just the torture tests.

```suggestion
to hook into the LLVM Test Suite system. These include GCC's C Torture tests and gfortran's test suite.
```

https://github.com/llvm/llvm-project/pull/128937