Added read-only testing to backends

This PR adds a flag to the test method so we can choose to test for read-only access (needed for restore) or read-write (needed for backups).

Additionally, this PR fixes a logic bug in that the BackendSourceProvider would not actually test the connection, but just check the placeholder element returned (which is always returned). With this PR the backend's ListAsync command is tested fully.

Also renamed the methods on ISourceProvider to have the `...Async` prefix.

This replaces #6939

proprietary/GoogleWorkspace/WebModule/WebModule.cs

@@ -95,9 +95,9 @@ public class WebModule : IWebModule
             forwardoptions[key!] = uri.QueryParameters[key];
 
         using var client = new SourceProvider(url, "", forwardoptions, true);
-        await client.Initialize(cancellationToken);
+        await client.InitializeAsync(cancellationToken);
 
-        var targetEntry = await client.GetEntry((path ?? "").TrimStart('/'), isFolder: true, cancellationToken).ConfigureAwait(false);
+        var targetEntry = await client.GetEntryAsync((path ?? "").TrimStart('/'), isFolder: true, cancellationToken).ConfigureAwait(false);
         if (targetEntry == null)
             throw new DirectoryNotFoundException($"Path not found: {path}");