[Flang][Tests] Add GPL notice to GFortran test suite documentation. (#161912)

Add a GPL notice to the GFortran test suite documentation and redirect
to the LICENSE file distributed with the test suite.

Co-authored-by: Cameron McInally <[email protected]>

Author: mcinally

flang/docs/FortranLLVMTestSuite.md

@@ -73,3 +73,5 @@ instructions described [above](#running-the-llvm-test-suite-with-fortran).
 There are additional configure-time options that can be used with the gfortran 
 tests. More details about those options and their purpose can be found in 
 [`Fortran/gfortran/README.md`](https://github.com/llvm/llvm-test-suite/tree/main/Fortran/gfortran/README.md).
+
+ These tests are Free Software and are shared under the terms of the GNU General Public License (GPL). For more details, please see the accompanying [`LICENSE`](https://github.com/llvm/llvm-test-suite/tree/main/Fortran/gfortran/LICENSE.txt) file.