Use %v when formatting objects in assert.Len

[benjaminjkraft]

Summary

Fix assert.Len's error message for non-%sable values.

Details

It was using %s, which doesn't work if the thing being len'ed is not a []string or similar. For example, assert.Len(t, []int{1}, 0) says that [%!s(int=1)] should have 1 item(s), but has 0. Now it uses %v which will work for almost anything.

[wwade]

@glesica @boyan-soubachov @mvdkleijn could someone please have a look at this one?

[dolmen]

@benjaminjkraft @k1ng440 @wwade Could you instead review #1485 which I plan to merge soon?