[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.

dev/test-dependencies.sh

@@ -79,7 +79,7 @@ for HADOOP_PROFILE in "${HADOOP_PROFILES[@]}"; do
   echo "Generating dependency manifest for $HADOOP_PROFILE"
   mkdir -p dev/pr-deps
   $MVN $HADOOP2_MODULE_PROFILES -P$HADOOP_PROFILE dependency:build-classpath -pl assembly \
-    | grep "Building Spark Project Assembly" -A 5 \
+    | grep "Dependencies classpath:" -A 1 \
     | tail -n 1 | tr ":" "\n" | rev | cut -d "/" -f 1 | rev | sort \
     | grep -v spark > dev/pr-deps/spark-deps-$HADOOP_PROFILE
 done