[SPARK-12712] Fix failure in ./dev/test-dependencies when run against empty .m2 cache

This patch fixes a bug in `./dev/test-dependencies.sh` which caused spurious failures when the script was run on a machine with an empty `.m2` cache. The problem was that extra log output from the dependency download was conflicting with the grep / regex used to identify the classpath in the Maven output. This patch fixes this issue by adjusting the regex pattern.

Tested manually with the following reproduction of the bug:

```
rm -rf ~/.m2/repository/org/apache/commons/
./dev/test-dependencies.sh
```

Author: Josh Rosen <joshrosen@databricks.com>

Closes #13568 from JoshRosen/SPARK-12712.