api test: Fix a test to use Account.copyWith when it means that

The "auth headers sent by default" test below assumes that the
request is made with the email and API key of `eg.selfAccount`.
It happens to be true that `eg.account(user: eg.selfUser, …)`
will get that email, but that's a bit of an accident about
how eg.account works and what data is on eg.selfUser.

Instead of `eg.account`, use `eg.selfAccount.copyWith`, to make
the intention more explicit.

Author: gnprice

test/api/core_test.dart

@@ -24,8 +24,7 @@ void main() {
     Future<http.BaseRequest> makeRequest(String realmUrl, String requestUrl, {
       bool? useAuth,
     }) {
-      final account = eg.account(user: eg.selfUser, apiKey: eg.selfAccount.apiKey,
-        realmUrl: Uri.parse(realmUrl));
+      final account = eg.selfAccount.copyWith(realmUrl: Uri.parse(realmUrl));
       return FakeApiConnection.with_(account: account, (connection) async {
         connection.prepare(json: {});
         final request = http.Request('GET', Uri.parse(requestUrl));