ash: add tests for failures of the exec and command builtins

ash: add tests for failures of the exec and command builtins

The exec builtin should return an exit status of 127 if the command
can't be found.  It doesn't:  it returns 2.

If the command builtin is used to source a script that runs a second
script that doesn't exist ash should issue an error.  Instead it seg
faults.

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>