test: run test suite with new_streaming=True for Polars lazy

[MarcoGorelli]

Similarly to

narwhals/tests/utils.py

Lines 86 to 89 in 04274b3

	if result.implementation is Implementation.POLARS and os.environ.get(
	"NARWHALS_POLARS_GPU", False
	): # pragma: no cover
	result = result.to_native().collect(engine="gpu")

we should also have an environment variable to run the tests with Polars' new streaming engine. This can help with finding issues such as pola-rs/polars#20930