cmd/gogio: rip out cgo deps for x11

Instead rely on more tiny standalone tools. In this case, scrot lets us
take png screenshots, and works well.

On the plus side, we remove some large X Go deps, and we don't need
nearly as much code.

While at it, skip if any of the tools are missing, and actually defer
the cleanup funcs so that they run when we fail the test early.

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

cmd/go.sum

@@ -1,14 +1,8 @@
 dmitri.shuralyov.com/gpu/mtl v0.0.0-20190408044501-666a987793e9/go.mod h1:H6x//7gZCb22OMCxBHrMx7a5I7Hp++hsVxbQ4BYO7hU=
 gioui.org v0.0.0-20191030194120-b53c2a7c9d56 h1:ZC5qDUrd0+64qlceTlMC1DM3bjEqAMohfP+3aE3U5OE=
 gioui.org v0.0.0-20191030194120-b53c2a7c9d56/go.mod h1:KqFFi2Dq5gYA3FJ0sDOt8OBXoMsuxMtE8v2f0JExXAY=
-github.com/BurntSushi/freetype-go v0.0.0-20160129220410-b763ddbfe298 h1:1qlsVAQJXZHsaM8b6OLVo6muQUQd4CwkH/D3fnnbHXA=
-github.com/BurntSushi/freetype-go v0.0.0-20160129220410-b763ddbfe298/go.mod h1:D+QujdIlUNfa0igpNMk6UIvlb6C252URs4yupRUV4lQ=
-github.com/BurntSushi/graphics-go v0.0.0-20160129215708-b43f31a4a966 h1:lTG4HQym5oPKjL7nGs+csTgiDna685ZXjxijkne828g=
-github.com/BurntSushi/graphics-go v0.0.0-20160129215708-b43f31a4a966/go.mod h1:Mid70uvE93zn9wgF92A/r5ixgnvX8Lh68fxp9KQBaI0=
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 github.com/BurntSushi/xgb v0.0.0-20160522181843-27f122750802/go.mod h1:IVnqGOEym/WlBOVXweHU+Q+/VP0lqqI8lqeDx9IjBqo=
-github.com/BurntSushi/xgbutil v0.0.0-20190907113008-ad855c713046 h1:O/r2Sj+8QcMF7V5IcmiE2sMFV2q3J47BEirxbXJAdzA=
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 github.com/chromedp/cdproto v0.0.0-20191009033829-c22f49c9ff0a h1:AuIGvB6IuWpMEdfKQ+t77D6dzLpNftzxAsktehYyWn8=
 github.com/chromedp/cdproto v0.0.0-20191009033829-c22f49c9ff0a/go.mod h1:PfAWWKJqjlGFYJEidUM6aVIWPr0EpobeyVWEEmplX7g=
 github.com/chromedp/chromedp v0.5.1 h1:PAqhoCWCHzRphYnmmxLSiYk7EEwDplCm4woTCCaV2cQ=