dirrotate: Test using awk rather than bc

[Lestropie]

Should fix issue reported in #3012. Seems to be some kind of race condition in the BASH [[ ]] / [ ] implementation, where sometimes the "1" yielded by bc would be invoked as a command name. But it was inconsistent. And running the script manually rather than via ctest would never fault. So kinda weird. I've verified that this is indeed testing for expected behaviour by flipping the inequality test.

[Lestropie]

Ie. This line of the test:

[[ $(echo "${b} < ${a}" | bc) -eq 1 ]]

Would yield:

(standard_in) 1: syntax error
CMake Error at /home/runner/work/mrtrix3/mrtrix3/cmake/RunTest.cmake:17 (message):
  Test
  /home/runner/work/mrtrix3/mrtrix3/testing/binaries/tests/dirrotate/default
  failed!

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"