test: solve 3 warnings about track expression in collapsible tree component

[davidlj95]

'NG0955: The provided track expression resulted in duplicated keys for a given collection. Adjust the tracking expression such that it uniquely identifies all the items in the collection. Duplicated keys were: 
key "" at index "0" and "1". Find more at https://angular.dev/errors/NG0955'
'NG0955: The provided track expression resulted in duplicated keys for a given collection. Adjust the tracking expression such that it uniquely identifies all the items in the collection. Duplicated keys were: 
key "" at index "0" and "1". Find more at https://angular.dev/errors/NG0955'
'NG0955: The provided track expression resulted in duplicated keys for a given collection. Adjust the tracking expression such that it uniquely identifies all the items in the collection. Duplicated keys were: 
key "" at index "0" and "1". Find more at https://angular.dev/errors/NG0955'