[SPARK-10359][PROJECT-INFRA] Use more random number in dev/test-dependencies.sh; fix version switching

This patch aims to fix another potential source of flakiness in the `dev/test-dependencies.sh` script.

pwendell's original patch and my version used `$(date +%s | tail -c6)` to generate a suffix to use when installing temporary Spark versions into the local Maven cache, but this value only changes once per second and thus is highly collision-prone when concurrent builds launch on AMPLab Jenkins. In order to reduce the potential for conflicts, this patch updates the script to call Python's random number generator instead.

I also fixed a bug in how we captured the original project version; the bug was causing the exit handler code to fail.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #10558 from JoshRosen/build-dep-tests-round-3.