fix failing build on hosts with wildcard dns

on hosts which have wildcard dns domains in their local domain search
list, builds failed because "nosuch.host" could actually be resolved.

Since ".host" isn't a recommended TLD by RFC2606, we use ".invalid" now.
And since this is not enough to fix the problem, we use now absoulte
domain names (having a '.' at the end)

http/grab_test.go

@@ -106,7 +106,7 @@ func (s *GrabDownloaderSuite) TestDownload404(c *C) {
 }
 
 func (s *GrabDownloaderSuite) TestDownloadConnectError(c *C) {
-	c.Assert(s.d.Download(s.ctx, "http://nosuch.host/", s.tempfile.Name()),
+	c.Assert(s.d.Download(s.ctx, "http://nosuch.host.invalid./", s.tempfile.Name()),
 		ErrorMatches, ".*no such host")
 }
 
@@ -130,7 +130,7 @@ func (s *GrabDownloaderSuite) TestGetLength404(c *C) {
 }
 
 func (s *GrabDownloaderSuite) TestGetLengthConnectError(c *C) {
-	_, err := s.d.GetLength(s.ctx, "http://nosuch.host/")
+	_, err := s.d.GetLength(s.ctx, "http://nosuch.host.invalid./")
 
 	c.Assert(err, ErrorMatches, ".*no such host")
 }