Fix uninitialized memory read in table_test (#6980)

Summary:
When using parameterized tests, `gtest` sometimes prints the test
parameters. If no other printing method is available, it essentially
produces a hex dump of the object. This can cause issues with valgrind
with types like `TestArgs` in `table_test`, where the object layout has
gaps (with uninitialized contents) due to the members' alignment
requirements. The patch fixes the uninitialized reads by providing an
`operator<<` for `TestArgs` and also makes sure all members are
initialized (in a consistent order) on all code paths.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6980

Test Plan: `valgrind --leak-check=full ./table_test`

Reviewed By: siying

Differential Revision: D22045536

Pulled By: ltamasi

fbshipit-source-id: 6f5920ac28c712d0aa88162fffb80172ed769c32

table/table_test.cc

@@ -605,6 +605,17 @@ struct TestArgs {
   bool use_mmap;
 };
 
+std::ostream& operator<<(std::ostream& os, const TestArgs& args) {
+  os << "type: " << args.type << " reverse_compare: " << args.reverse_compare
+     << " restart_interval: " << args.restart_interval
+     << " compression: " << args.compression
+     << " compression_parallel_threads: " << args.compression_parallel_threads
+     << " format_version: " << args.format_version
+     << " use_mmap: " << args.use_mmap;
+
+  return os;
+}
+
 static std::vector<TestArgs> GenerateArgList() {
   std::vector<TestArgs> test_args;
   std::vector<TestType> test_types = {
@@ -662,6 +673,7 @@ static std::vector<TestArgs> GenerateArgList() {
         one_arg.restart_interval = restart_intervals[0];
         one_arg.compression = compression_types[0].first;
         one_arg.compression_parallel_threads = 1;
+        one_arg.format_version = 0;
         one_arg.use_mmap = true;
         test_args.push_back(one_arg);
         one_arg.use_mmap = false;
@@ -678,8 +690,8 @@ static std::vector<TestArgs> GenerateArgList() {
             one_arg.reverse_compare = reverse_compare;
             one_arg.restart_interval = restart_interval;
             one_arg.compression = compression_type.first;
-            one_arg.format_version = compression_type.second ? 2 : 1;
             one_arg.compression_parallel_threads = num_threads;
+            one_arg.format_version = compression_type.second ? 2 : 1;
             one_arg.use_mmap = false;
             test_args.push_back(one_arg);
           }