Reduce the code in TestRequestMessageBase by extracting ExpandVariable.
Reduce the code in TestRequestMessageBase by extracting ExpandVariable.

R=csharp@chromium.org
BUG=

Review URL: https://codereview.appspot.com/69220043

madecoste/swarming,madecoste/swarming,luci/luci-py,pombreda/swarming,luci/luci-py,pombreda/swarming,pombreda/swarming,pombreda/swarming,luci/luci-py,madecoste/swarming,madecoste/swarming,luci/luci-py