Improve Java 7 support in build.xml:
- Check correctness of java.7.home and fail early
- Use ${java.home}/bin/java as a fallback
- Echo the path to the selected Java executable
- Simplify test-init1, test-init2 tasks.

I hope that this fixes "Unsupported major.minor version 52.0" errors on Gump.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/tc7.0.x/trunk@1640708 13f79535-47bb-0310-9956-ffa450edef68