Fix diff command to only set --strip-trailing-cr if available

[thbeu]

As reported by #111 (comment).

[landryb]

thanks ! fwiw there are other occurences in test1/2/3.. i'll do another PR on top to change the shebang to plain sh, there's no need for bash here

[thbeu]

i'll do another PR on top to change the shebang to plain sh, there's no need for bash here

Have a look at #150 first.

[landryb]

thanks, with #150 & #152, all tests for 1.6.1 pass fine on OpenBSD

100% tests passed, 0 tests failed out of 7