Warnings on building Testbed from uses of *sprintf

[louis-langholtz]

Expected/Desired Behavior or Experience:

Testbed isn't using *sprintf functions having security concerns over their design.

Actual Behavior:

Testbed is using various *sprintf functions. Some compilers, like clang and gcc, issue warnings like the following about this:

PlayRho/Testbed/Framework/Test.cpp: In member function 'void testbed::Test::Step(const testbed::Settings&, testbed::Drawer&, UiState&)':
PlayRho/Testbed/Framework/Test.cpp:1274:21: warning: 'int sprintf(char*, const char*, ...)' is deprecated: This function is provided for compatibility reasons only.  Due to security concerns inherent in the design of sprintf(3), it is highly recommended that you use snprintf(3) instead. [-Wdeprecated-declarations]
 1274 |         std::sprintf(buffer, "Max of %u", m_stats.m_maxTouching);
      |         ~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Steps to Reproduce the Actual Behavior:

git clone --recurse-submodules https://github.com/louis-langholtz/PlayRho.git
mkdir PlayRho-build
cd PlayRho-build
cmake -DPLAYRHO_BUILD_TESTBED=ON ../PlayRho
make