[dev.garbage] runtime: Linear map test give false negative due to GC.

[dev.garbage] runtime: Linear map test give false negative due to GC.
This test gives a false negative at an observed rate of 1 in a 1000
due to the fact that it runs for < 100 ms. allowing GC pauses to
warp the results. Changed the test so that it triggers only if it
remains non-linear for much larger problem sizes.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/164010043