Fix Maven scalatest filereports path:

The filename listed in <filereports> is placed under the <reportsDirectory>;
the current code places it in a subdirectory of reportsDirectory, e.g.

${project.build.directory}/surefire-reports/${project.build.directory}/SparkTestSuite.txt

This caused problems under Windows because it would try to create
a subdirectory with a colon in its name.