The Maven build was failing on Windows because it tried to call the unix `unzip` utility to extract the Py4J files into core's build directory.  I've fixed this issue by using the `maven-antrun-plugin` to perform the unzipping.

I also fixed an issue that prevented tests from running under Windows:

In the Maven ScalaTest plugin, 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 named "c:\\".

Note that the tests still fail under Windows (for other reasons); this PR just allows them to run and fail rather than crash when trying to create the test reports directory.

Author: Josh Rosen <joshrosen@apache.org>
Author: Josh Rosen <rosenville@gmail.com>
Author: Josh Rosen <joshrosen@databricks.com>

Closes #2165 from JoshRosen/windows-support and squashes the following commits:

651d210 [Josh Rosen] Unzip to python/build instead of core/build
fbf3e61 [Josh Rosen] 4 spaces -> 2 spaces
e347668 [Josh Rosen] Fix Maven scalatest filereports path:
4994af1 [Josh Rosen] [SPARK-3061] Use maven-antrun-plugin to unzip Py4J.