[SPARK-3061] Fix Maven build under Windows

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.
[SPARK-1919] Fix Windows spark-shell --jars

We were trying to add `file:/C:/path/to/my.jar` to the class path. We should add `C:/path/to/my.jar` instead. Tested on Windows 8.1.

Author: Andrew Or <andrewor14@gmail.com>

Closes #2211 from andrewor14/windows-shell-jars and squashes the following commits:

262c6a2 [Andrew Or] Oops... Add the new code to the correct place
0d5a0c1 [Andrew Or] Format jar path only for adding to shell classpath
42bd626 [Andrew Or] Remove unnecessary code
0049f1b [Andrew Or] Remove embarrassing log messages
b1755a0 [Andrew Or] Format jar paths properly before adding them to the classpath