Fix RTprop overestimation due to off-by-one timestamp diff.

6 jobs for develop in 21 seconds (queued for 2 seconds)
Name Stage Failure
failed
build:prrt Build There is an unknown failure, please try again
  atomic_int items;
^
tests/CMakeFiles/prrtTests.dir/build.make:86: recipe for target 'tests/CMakeFiles/prrtTests.dir/receptionTable_tests.cpp.o' failed
make[2]: *** [tests/CMakeFiles/prrtTests.dir/receptionTable_tests.cpp.o] Error 1
CMakeFiles/Makefile2:398: recipe for target 'tests/CMakeFiles/prrtTests.dir/all' failed
make[1]: *** [tests/CMakeFiles/prrtTests.dir/all] Error 2
Makefile:138: recipe for target 'all' failed
make: *** [all] Error 2
ERROR: Job failed: exit status 1