test(python): Improved import polars timing test; now much more consistent/reliable

[alexander-beedie]

Ref: #12094 (comment).

Improves the test in several ways:

• Adds an up-front .py → .pyc step (fast, only runs once).
• Factors-out (and fixes!) the "cumulative_time" sort that chooses the fastest run.
• Early-exits from the test as soon as an import run meets the max allowed time.

Should be much more reliable now.

(Fastest time I saw while testing was ~0.059 secs, so initial import remains extremely fast).

[stinodego]

Thanks! <3