buptest: base testing subproc funcs ex() and exo() on run()

Rework the subprocess functions, providing ex() and exo() as concise
test functions that print the commands they're executing.  Base them
on a common, lower-level run() function.

Drop exc() since ex() and exo() both check the exit status by default.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>