tests: improve setarch tests, regarding --uname-2.6 issues

tests: improve setarch tests, regarding --uname-2.6 issues

Add some more tests:
  * error handling "unknown arch" and "unknown command"
  * "noop" test with host arch and no other options
  * all options except --uname-2.6
  * --uname-2.6 whithout any other options but handle fatal
    glibc error "kernel too old" (with debug output)
  * add a "real" --uname-2.6 test which validates uname(1)
    output

Note the "kernel too old" cases are systems where glibc was
configured to support only kernels > 3.0. On some archs
(e.g. x86) --uname-2.6 still works with such glibc because
2.6.39 had all needed features on board (e.g. vdso).
See https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=806911

Some people reported segfaults (after execvp) but I can only
reproduce it on arm* and aarch64 qemu-user-space builds. We
don't need to fix our tests for such broken systems where
also many other tests fail currently.

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>