sharness tests: output the problem

[jbenet]

it would be nice if the sharness tests output the problem when they failed.

perhaps the way to do it is to have a command (per test case) that only gets run when the test case fails.

For example:

test_expect_success "ipfs init output looks good" '
    STARTHASH="QmYpv2VEsxzTTXRYX3PjDg961cnJE3kY1YDXLycHGQ3zZB" &&
    echo "initializing ipfs node at $IPFS_DIR" >expected &&
    echo "generating key pair...done" >>expected &&
    echo "peer identity: $PEERID" >>expected &&
    echo "\nto get started, enter: ipfs cat $STARTHASH" >>expected &&
    test_cmp expected actual_init
' '
    diff expected actual_init
'

this isn't the clearest thing ever, but may get the job done. this would require messing with sharness internals-- @chriscool any thoughts on easiest way to do this? or whether sharness already has a way of doing this?

[jbenet]

Addressed in #493