Provide mechanism to restart tests from previous error

Summary:
While running rocksdb tests, we sometimes encounter errors and
the test run stops. We now provide a new make target call check_some
which restarts the test run from a specific test and continues from
there depending on the value of the environment variable ROCKSDBTESTS_START

Test Plan:
Run make check_some with different values of
ROCKSDBTESTS_START.

Reviewers: igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D29913

Makefile

@@ -161,6 +161,8 @@ TESTS = \
 	compaction_job_test \
 	thread_list_test
 
+SUBSET :=  $(shell echo $(TESTS) |sed s/^.*$(ROCKSDBTESTS_START)/$(ROCKSDBTESTS_START)/)
+
 TOOLS = \
         sst_dump \
 	db_sanity_test \
@@ -247,6 +249,10 @@ check: $(TESTS) ldb
 	for t in $(TESTS); do echo "***** Running $$t"; ./$$t || exit 1; done
 	python tools/ldb_test.py
 
+check_some: $(SUBSET) ldb
+	for t in $(SUBSET); do echo "***** Running $$t"; ./$$t || exit 1; done
+	python tools/ldb_test.py
+
 ldb_tests: ldb
 	python tools/ldb_test.py