add an env var ROCKSDB_TESTS_FROM to control where to start from a list of tests

Summary:
Sometimes, I got a test failure. After fixing that, I want to resume
db_test from that test. ROCKSDB_TESTS_FROM is for this purpose.

Test Plan: as title

Reviewers: yhchiang, rven, igor, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D27807

util/testharness.cc

@@ -41,18 +41,29 @@ bool RegisterTest(const char* base, const char* name, void (*func)()) {
 int RunAllTests() {
   port::InstallStackTraceHandler();
 
-  const char* matcher = getenv("ROCKSDB_TESTS");
+  const char* one_matcher = getenv("ROCKSDB_TESTS");
+  const char* from_matcher = getenv("ROCKSDB_TESTS_FROM");
 
   int num = 0;
+  bool tests_on = (one_matcher == nullptr && from_matcher == nullptr);
   if (tests != nullptr) {
     for (unsigned int i = 0; i < tests->size(); i++) {
       const Test& t = (*tests)[i];
-      if (matcher != nullptr) {
-        std::string name = t.base;
-        name.push_back('.');
-        name.append(t.name);
-        if (strstr(name.c_str(), matcher) == nullptr) {
-          continue;
+      if (tests_on == false) {
+        if (one_matcher != nullptr || from_matcher != nullptr) {
+          std::string name = t.base;
+          name.push_back('.');
+          name.append(t.name);
+          if (from_matcher != nullptr &&
+              strstr(name.c_str(), from_matcher) != nullptr) {
+            tests_on = true;
+          }
+          if (!tests_on) {
+            if (one_matcher == nullptr ||
+                strstr(name.c_str(), one_matcher) == nullptr) {
+              continue;
+            }
+          }
         }
       }
       fprintf(stderr, "==== Test %s.%s\n", t.base, t.name);

util/testharness.h

@@ -21,9 +21,11 @@ namespace rocksdb {
 namespace test {
 
 // Run some of the tests registered by the TEST() macro.  If the
-// environment variable "ROCKSDB_TESTS" is not set, runs all tests.
-// Otherwise, runs only the tests whose name contains the value of
-// "ROCKSDB_TESTS" as a substring.  E.g., suppose the tests are:
+// environment variable "ROCKSDB_TESTS" and "ROCKSDB_TESTS_FROM"
+// are not set, runs all tests. Otherwise, run all tests after
+// ROCKSDB_TESTS_FROM and those specified by ROCKSDB_TESTS.
+// Partial name match also works for ROCKSDB_TESTS and
+// ROCKSDB_TESTS_FROM. E.g., suppose the tests are:
 //    TEST(Foo, Hello) { ... }
 //    TEST(Foo, World) { ... }
 // ROCKSDB_TESTS=Hello will run the first test