build: print the error result when the tests fail

Any diagnostic information is useful when things fail.  In my case, it
printed out the fact that the tests were segfaulting.
build: add failure message along with number in GenerateHelptags.cmake
Merge pull request #1544 from jszakmeister/run-tests-error-code

build: print the error result when the tests fail
Linting: Integrate in CMake, enable by default.
Linting: Fix header guards, add whitespace.
Linting: fix strcpy warning.
Linting: Suppress warnings in os/users.c.