add samples for listAssertions of api-infix

[robstoll]

Platform (all, jvm, js, android): all
Extension (none, kotlin 1.3): none

Code related feature

We also want to provide samples for the infix api

Following the things you need to do:

• copy ListAssertionsSample from fluent-api, adjust imports ...api.fluent... to ...api.infix... and ajdust the samples accordingly (always use the infix syntax, never the fluent).
• check that the sample in the KDoc Intellij shows looks good (ALT+Mouse-click on function name)

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[lilimapradhan9]

I'll work on this