Disable flaky test.
Disable flaky test.

vpython/venv's TestVirtualEnv seems to be consistently flaky on builders.
Disable it until this can be resolved.

TBR=iannucci@chromium.org
BUG=chromium:691818
TEST=None

Review-Url: https://codereview.chromium.org/2743523006

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go