test: `ctime_tests` usage always suggests using libtool

[fanquake]

It will probably be suprising for someone building binaries using CMake to be recommened to run them under libtool:

cmake -B build
cmake --build build
./build/bin/ctime_tests 
This test can only usefully be run inside valgrind because it was not compiled under msan.
Usage: libtool --mode=execute valgrind ./ctime_tests