added utilities/string/chrono.h; fix test-waiting-on.cpp to use it

[maloel]

Durations do not have an operator<< unless you're in C++20, so output from test-waiting-on was not readable.
Before:

C:\jenkins_sys_rsbuild\workspace\LRS_windows_compile_pipeline\unit-tests\utilities\time\test-waiting-on.cpp(164): FAILED:
  REQUIRE( wait_time >= timeout )
with expansion:
  {?} >= {?}

After:

with expansion:
  0.005s > 1.99s

[Nir-Az]

Looks good!