cmd/cover: remove use of diff in cover_test.go

It's non-portable, and the test isn't hard to write without diff.
It still produces helpful output in case of trouble:

--- FAIL: TestCoverHTML (0.75s)
    cover_test.go:325: line 4 differs: got:
        	case &lt;-ch:<span class="cov0" title="0"></span>
        want:
        	case &lt;-ch:<span class="cov0" xitle="0"></span>

This makes the test operating-system independent.

Change-Id: Iff35f00cb76ba89bc1b93db01c6f994e74341f4a
Reviewed-on: https://go-review.googlesource.com/118795


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>