Add example xml file for WilsonFlow test

[vataspro]

Add an example xml file for WilsonFlow test in tests/smearing/Test_WilsonFlow.cc.

[edbennett]

It'll look something like this:

 <grid>
  <WilsonFlow>
    <steps>12800</steps>
    <step_size>0.01</step_size>
    <meas_interval>10</meas_interval>
  </WilsonFlow>
  <Configurations>
    <conf_prefix>../runRUN/ckpoint_lat</conf_prefix>
    <rng_prefix>../runRUN/ckpoint_rng</rng_prefix>
    <StartConfiguration>START</StartConfiguration>
    <EndConfiguration>END</EndConfiguration>
    <Skip>32</Skip>
  </Configurations>
 </grid>