Run the GeoJS lookup tests on a clock the test controls (closes #73)

The tests that have GeoJS asked now run in a synctest bubble, so a wait
lasts exactly as long as it should however slowly the test process runs:
a new client's wait is checked to be exactly one second, and its next
request exactly no wait. A request waiting on the network would stop the
bubble's clock, so the stand-in for GeoJS now answers in place of the
network, through a transport that a test-only file lets the tests set.
The test of the failure log uses an abandoned request instead of a
closed port.

Model: opus-5-5

internal/lookup/export_test.go

@@ -0,0 +1,9 @@
+package lookup
+
+import "net/http"
+
+// SetTransport has g's requests to GeoJS go through transport instead of
+// the network.
+func (g *GeoJS) SetTransport(transport http.RoundTripper) {
+	g.httpClient.Transport = transport
+}