wvtool: Fix JUnix XML generation

Now, the resulting XML passes
  xmllint --schema http://windyroad.com.au/dl/Open%20Source/JUnit.xsd
wvtool: Escape values put into JUnit XML
wvtool: Briefly document wvjunit.py
wvtool: Reformat check lines in JUnit XML output

Previously, the output looked like this:

    <failure type="" message="">! Testing "my error test function" in mytest.t.cc:
    ! mytest.t.cc.433     3 &lt; 4  FAILED
    </failure>

and now it looks like (note the dots before "FAILED"):

    <failure type="" message="">! Testing "my error test function" in mytest.t.cc:
    ! mytest.t.cc.433     3 &lt; 4 .......................................... FAILED
    </failure>