fixed a bug where my $_ was getting clobbered (apparently)

also changed a variable name to be more self-documenting
also fixed a stray $_ that was just totally wrong
new switch: --default_yn (lets you control what the default for yes/no prompts will be)

default value is 'n' (which is consistent with the previous behavior
this is most useful when controlled by environment variable (e.g. VCTOOLS_DEFAULT_YN=y)
three allowed values: "y", "n", or "off"
-	"off" means don't accept a default at all
also tweaked the test output stuff to use Test::Trap's diag_all method, which I recently discovered
also had to fiddle with test_prompt_input for some reason
-	I still don't understand exactly _why_ it works, but it works