Fix target 'clean' to include parallel test binaries (#5629)

Summary:
current `clean` target in Makefile does not remove parallel test
binaries. Fix this.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5629

Test Plan:
(on devserver)
Take file_reader_writer_test for instance.
```
$make -j32 file_reader_writer_test
$make clean
```
Verify that binary file 'file_reader_writer_test' is delete by `make clean`.

Differential Revision: D16513176

Pulled By: riversand963

fbshipit-source-id: 70acb9f56c928a494964121b86aacc0090f31ff6

Makefile

@@ -1068,7 +1068,7 @@ rocksdb.h rocksdb.cc: build_tools/amalgamate.py Makefile $(LIB_SOURCES) unity.cc
 	build_tools/amalgamate.py -I. -i./include unity.cc -x include/rocksdb/c.h -H rocksdb.h -o rocksdb.cc
 
 clean:
-	rm -f $(BENCHMARKS) $(TOOLS) $(TESTS) $(LIBRARY) $(SHARED)
+	rm -f $(BENCHMARKS) $(TOOLS) $(TESTS) $(PARALLEL_TEST) $(LIBRARY) $(SHARED)
 	rm -rf $(CLEAN_FILES) ios-x86 ios-arm scan_build_report
 	$(FIND) . -name "*.[oda]" -exec rm -f {} \;
 	$(FIND) . -type f -regex ".*\.\(\(gcda\)\|\(gcno\)\)" -exec rm {} \;