wordexp: Split out command execution tests from posix/wordexp-test

Once wordexp switches to posix_spawn, testing for command execution
based on fork handlers will not work anymore.  Therefore, move these
subtests into  a new test, posix/tst-wordexp-nocmd, which uses a
different form of command execution detection, based on PID
namespaces.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>