fix compiler warnings for CPP tests

[jameslamb]

The C++ tests currently raise the following compiler warnings with clang 11:

tests/cpp_tests/test_byte_buffer.cpp:33:21: warning: comparison of integers of different signs: 'int' and 'unsigned long' [-Wsign-compare]
  for (int i = 0; i < sizeof(int16_t); i++) {
                  ~ ^ ~~~~~~~~~~~~~~~
tests/cpp_tests/test_byte_buffer.cpp:44:21: warning: comparison of integers of different signs: 'int' and 'unsigned long' [-Wsign-compare]
  for (int i = 0; i < sizeof(int64_t); i++) {
                  ~ ^ ~~~~~~~~~~~~~~~
tests/cpp_tests/test_byte_buffer.cpp:55:21: warning: comparison of integers of different signs: 'int' and 'unsigned long' [-Wsign-compare]
  for (int i = 0; i < sizeof(double); i++) {
  

tests/cpp_tests/testutils.cpp:268:25: warning: unqualified call to 'std::move' [-Wunqualified-std-cast-call]
      threads.push_back(move(th));

(build link)

This PR fixes them.

[github-actions]

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