First, add a headless boolean flag that defaults to true. That way, one
can run 'go test -headless=false' to, for example, see how Chrome runs
the webassembly endtoend test.
Second, skip the Chrome test if the browser isn't installed.
While at it, run 'gofmt -s' on the package.
Signed-off-by: Daniel Martí <mvdan@mvdan.cc>