Add LICENSE file, along with a copy of the GPLv3.

Closes Trac #8117.
Signed-off-by: Austin Seipp <aseipp@pobox.com>

testsuite/LICENSE

+The Glasgow Haskell Compiler License
+
+Copyright 2002, The University Court of the University of Glasgow. 
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+- Redistributions of source code must retain the above copyright notice,
+this list of conditions and the following disclaimer.
+ 
+- Redistributions in binary form must reproduce the above copyright notice,
+this list of conditions and the following disclaimer in the documentation
+and/or other materials provided with the distribution.
+ 
+- Neither name of the University nor the names of its contributors may be
+used to endorse or promote products derived from this software without
+specific prior written permission. 
+
+THIS SOFTWARE IS PROVIDED BY THE UNIVERSITY COURT OF THE UNIVERSITY OF
+GLASGOW AND THE CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
+INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
+FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+UNIVERSITY COURT OF THE UNIVERSITY OF GLASGOW OR THE CONTRIBUTORS BE LIABLE
+FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH
+DAMAGE.
+
+================================================================================
+The GHC testsuite contains several files under different licenses, explained
+here:
+
+ The following tests are under the GPLv3, or (at your option) any later version:
+
+  - tests/indexed-types/should_compile/T3787.hs
+  - tests/typecheck/should_compile/T4524.hs
+  - tests/simplCore/should_run/T3591.hs
+
+ You should have received a copy of the GNU General Public License along with
+ the GHC testsuite, in LICENSE.GPL