[server] Fix race in the logging test.
[server] Fix race in the logging test.

Sometimes the logging event about the admin port starting happened after
the main port has already served the test request, which caused unexpected
ordering of logging events.

Wait for the both ports to start serving before issuing any test requests.

R=ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org, tandrii@chromium.org

Change-Id: Ia2321d11b8228c8848925b59d03adb099bea0c26
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/1954752
Reviewed-by: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Reviewed-by: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go