tests: Use shell in local cmd_execute() only if needed

tests: Use shell in local cmd_execute() only if needed

The generic cmd_execute() function was introduced in a manner that
converted the argument array to a string and used shell to run the
command unconditionally. This is not really desirable, so move back to
using the command array by default and use the single command string
with a shell only when really needed.

Signed-off-by: Jouni Malinen <j@w1.fi>