cmd/gogio: add the first end-to-end X11 test

Right now it's very similar to the JS test on Chrome. Like it, this one
just runs the "red.go" gio app, takes a screenshot, and expects to see
red.

It also supports the -headless flag; when true, Xvfb is used and it's
entirely headless and hidden. Otherwise, Xephyr is used and once can see
the test in action. If the tool isn't installed, the test is skipped.

We need to add xgb as a dependency, so that we can connect to the X
server and interact with it, like taking screenshots.

Finally, this is an initial version, and a number of TODOs are left for
a later time. They'll get fixed in follow-up patches.

While at it, start making all tests parallel, since the end-to-end tests
take about a second each and neither are very cpu-intensive.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>

cmd/go.mod

@@ -4,6 +4,10 @@ go 1.13
 
 require (
 	gioui.org v0.0.0-20191029130630-4641607cd6a4
+	github.com/BurntSushi/freetype-go v0.0.0-20160129220410-b763ddbfe298 // indirect
+	github.com/BurntSushi/graphics-go v0.0.0-20160129215708-b43f31a4a966 // indirect
+	github.com/BurntSushi/xgb v0.0.0-20160522181843-27f122750802
+	github.com/BurntSushi/xgbutil v0.0.0-20190907113008-ad855c713046
 	github.com/chromedp/cdproto v0.0.0-20191009033829-c22f49c9ff0a
 	github.com/chromedp/chromedp v0.5.1
 	golang.org/x/image v0.0.0-20190802002840-cff245a6509b