Remove existing positional args when running a single test on pytest

[kimadeline]

For #5757

Positional arguments are not always the last ones in the args list: in the repro case in #5757 (comment) the arg list is [ "tests", "-sv"]. This PR makes the parsing of positional args more flexible: instead of slicing the args array and keeping everything after the last option, we filter out non-positional args.

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