Difference in time should use difftime not subtraction

[HughParsonage]

tinytest/pkg/R/tinytest.R

Line 698 in c3ddc5b

td <- abs(Sys.time() - t0)

This gives the wrong result for some durations

cat(tinytest:::humanize(abs(Sys.time() - (Sys.time() - 60 * 4.1))))
#> 4.1s

cat(tinytest:::humanize(difftime(Sys.time(), Sys.time() - 60 * 4.1, units = "secs")))
#> 4m 6.0s

(The abs should be removed too.)