Split benchmarks and test tools into separate targets.

This lets us remove main() from snappy_bench.cc and snappy_unittest.cc,
which simplifies integrating these tests and benchmarks with other
suites.

PiperOrigin-RevId: 347857427

Author: pwnall

.appveyor.yml

@@ -34,3 +34,4 @@ build_script:
 
 test_script:
   - build\%CONFIGURATION%\snappy_unittest
+  - build\%CONFIGURATION%\snappy_benchmark

.travis.yml

@@ -90,6 +90,7 @@ before_script:
 
 script:
 - build/snappy_unittest
+- build/snappy_benchmark
 - if [ -f build/snappy_compress_fuzzer ]; then
     build/snappy_compress_fuzzer -runs=1000 -close_fd_mask=3;
   fi