Fold testsuite.git into ghc.git (re #8545)

This commit performs a subtree merge of testsuite.git into ghc.git;
The next commit will adapt `sync-all` et al. to the new situation.

At the time of merge, testsuite.git was at commit
[998a816ae89c4fd573f4abd7c6abb346cf7ee9af/testsuite]

The following steps have been used to accomplish this merge:

1. Clone a fresh testsuite.git copy (& cd into)

2. Remove accidentally committed binary files from history

 git filter-branch \
  --index-filter "git rm -r --cached --ignore-unmatch \
    tests/haddock/should_compile_flag_nohaddock/a.out \
    tests/haddock/should_compile_noflag_nohaddock/a.out \
    tests/ghc-regress/haddock/should_compile_flag_nohaddock/a.out \
    tests/ghc-regress/haddock/should_compile_noflag_nohaddock/a.out \
    tests/ghc-regress/dph/diophantine/dph-diophantine-fast \
    tests/ghc-regress/dph/diophantine/dph-diophantine-opt \
    tests/ghc-regress/dph/primespj/dph-primespj-fast \
    tests/ghc-regress/dph/quickhull/dph-quickhull-fast \
    tests/ghc-regress/dph/smvm/dph-smvm \
    tests/ghc-regress/dph/sumnats/dph-sumnats \
    tests/ghc-regress/dph/words/dph-words-fast \
    tests/ghc-regress/plugins/plugins01" \
   HEAD

3. Rename all paths in testsuite.git to be prefixed with `testsuite/`

 git filter-branch -f --prune-empty --tree-filter \
  "mkdir -p testsuite; \
   git ls-tree --name-only \$GIT_COMMIT | xargs -I files mv files testsuite/"

4. cd into ghc/ checkout, and perform subtree merge of testsuite into ghc
   (see also http://nuclearsquid.com/writings/subtree-merging-and-you/

)

 cd ../ghc/
 git remote add -f testsuite ../testsuite/.git
 git merge -s ours --no-commit testsuite/master
 git read-tree --prefix=/ -u testsuite/master
 git commit

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>