runtime: TestGcSys fails on Windows

[bradfitz]

--- FAIL: TestGcSys (0.06s)
    gc_test.go:27: expected "OK\n", but got "using too much memory: 39882752 bytes\n"
FAIL

Exposed by https://go-review.googlesource.com/122576 (for #23343)

[gopherbot]

Change https://golang.org/cl/130875 mentions this issue: runtime: skip TestGcSys on Windows

[gopherbot]

Change https://golang.org/cl/184097 mentions this issue: runtime: use hard heap goal if we've done more scan work than expected

[mknyszek]

This should be fixed now as of aae0b5b.