selftests/exec: allow shell return code of 126

When the shell fails to invoke a script because its path name
is too long (ENAMETOOLONG), most shells return 127 to indicate
command not found.  However, some systems report 126 (which POSIX
suggests should indicate a non-executable file) for this case,
so allow that too.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: David Drysdale <drysdale@google.com>
Tested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>