Rename bench-scripts test dir to pbench-agent

To help understand why and how the unit test environment looks, we
have renamed the test directory to pbench-agent, to be closer to
the name of the /var/lib/pbench-agent directory.

Author: portante

agent/bench-scripts/gold/pbench-dbench/test-02.txt

@@ -2,14 +2,14 @@
 ERROR: foo must be executable
 --- Finished test-19 pbench-fio (status=1)
 +++ pbench tree state
-/var/tmp/pbench-test-bench/pbench
-/var/tmp/pbench-test-bench/pbench/tmp
-/var/tmp/pbench-test-bench/pbench/tools-default
-/var/tmp/pbench-test-bench/pbench/tools-default/mpstat
-/var/tmp/pbench-test-bench/pbench/tools-default/sar
+/var/tmp/pbench-test-bench/pbench-agent
+/var/tmp/pbench-test-bench/pbench-agent/tmp
+/var/tmp/pbench-test-bench/pbench-agent/tools-default
+/var/tmp/pbench-test-bench/pbench-agent/tools-default/mpstat
+/var/tmp/pbench-test-bench/pbench-agent/tools-default/sar
 --- pbench tree state
 +++ pbench.log file contents
-grep: /var/tmp/pbench-test-bench/pbench/pbench.log: No such file or directory
+grep: /var/tmp/pbench-test-bench/pbench-agent/pbench.log: No such file or directory
 --- pbench.log file contents
 +++ test-execution.log file contents
 --- test-execution.log file contents