cloud.MockClient: Use Contains() to implement Search

The TFC API doesn't behave this way; a search term just signals the
substring was found within the name.

internal/cloud/tfe_client_mock.go

@@ -1103,7 +1103,7 @@ func (m *MockWorkspaces) List(ctx context.Context, organization string, options
 	// Get all the workspaces that match the prefix.
 	var ws []*tfe.Workspace
 	for _, w := range m.workspaceIDs {
-		if strings.HasPrefix(w.Name, prefix) {
+		if strings.Contains(w.Name, prefix) {
 			ws = append(ws, w)
 		}
 	}