test: warm up discovery before timing an empty scan

[ogorzalka]

Problem

DiscoveryCacheManagerSlowScanTest › says nothing about a location that scans quickly failed in most local pest --parallel runs (Method warning() … should be called exactly 0 times but called 1 times), never in serial runs or CI.

The test times a real scan of an empty directory against the 250 ms slow-scan threshold. Instrumenting getStructuresForLocation():

	first scan in the process	following scans
idle	150–225 ms	~1 ms
16 busy cores	252–421 ms	~1 ms

The first scan pays for loading the discoverer's classes; under parallel load that alone crosses the threshold. Reproduced in isolation: 3/10 failures with the CPU saturated.

Change

Test only: one warm-up scan of another empty directory before the measured one. The threshold and the product code are unchanged.

Result

0/15 failures with the CPU saturated; full suite in parallel 6/6 green (1367 tests).