tests: getexecutable: call `--version` on all Pythons
tests: getexecutable: call `--version` on all Pythons

This should prevent errors from pyenv:

    pyenv: python2.6: command not found

    The `python2.6' command exists in these Python versions:
      2.6.9

While the pyenv wrapper explicitly returns 127, I think it is better to
just check for non-zero?!

davidszotten/pytest,Akasurde/pytest,tareqalayan/pytest,flub/pytest,txomon/pytest,tgoodlet/pytest,pytest-dev/pytest,jaraco/pytest,tomviner/pytest,RonnyPfannschmidt/pytest,nicoddemus/pytest,hpk42/pytest,markshao/pytest,skylarjhdownes/pytest,vmalloc/dessert,The-Compiler/pytest,The-Compiler/pytest,JonathonSonesen/pytest,hpk42/pytest,eli-b/pytest,ddboline/pytest,pfctdayelise/pytest,tomviner/pytest,etataurov/pytest,MichaelAquilina/pytest,JonathonSonesen/pytest,alfredodeza/pytest,malinoff/pytest,rmfitzpatrick/pytest,nicoddemus/pytest,hackebrot/pytest