Add testing description

TESTING.md

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+Testing
+=======
+
+Zstandard CI testing is split up into three sections:
+short, medium, and long tests.
+
+Short Tests
+-----------
+Short tests run on CircleCI for new commits on every branch and pull request.
+They consist of the following tests:
+- Compilation on all supported targets (x86, x86_64, ARM, AArch64, PowerPC, and PowerPC64)
+- Compilation on various versions of gcc, clang, and g++
+- `tests/playTests.sh` on x86_64, without the tests on long data (CLI tests)
+- Small tests (`tests/legacy.c`, `tests/longmatch.c`, `tests/symbols.c`) on x64_64
+
+Medium Tests
+------------
+Medium tests run on every commit and pull request to `dev` branch, on TravisCI.
+They consist of the following tests:
+- The following tests run with UBsan and Asan on x86_64 and x86, as well as with
+  Msan on x86_64
+  - `tests/playTests.sh --test-long-data`
+  - Fuzzer tests: `tests/fuzzer.c`, `tests/zstreamtest.c`, and `tests/decodecorpus.c`
+- `tests/zstreamtest.c` under Tsan (streaming mode, including multithreaded mode)
+- Valgrind Test (`make -C tests valgrindTest`) (testing CLI and fuzzer under valgrind)
+- Fuzzer tests (see above) on ARM, AArch64, PowerPC, and PowerPC64
+
+Long Tests
+----------
+Long tests run on all commits to `master` branch,
+and once a day on the current version of `dev` branch,
+on TravisCI.
+They consist of the following tests:
+- Entire test suite (including fuzzers and some other specialized tests) on:
+  - x86_64 and x86 with UBsan and Asan
+  - x86_64 with Msan
+  - ARM, AArch64, PowerPC, and PowerPC64
+- Streaming mode fuzzer with Tsan (for the `zstdmt` testing)
+- ZlibWrapper tests, including under valgrind
+- Versions test (ensuring `zstd` can decode files from all previous versions)
+- `pzstd` with asan and tsan, as well as in 32-bits mode
+- Testing `zstd` with legacy mode off
+- Testing `zbuff` (old streaming API)
+- Entire test suite and make install on OS X