removed flawed test assertion

The test had two shortcomings:
- it combined two unrelated aspects
- it verified that files get removed that were not created during this
  test. So the test outcome was dependent on execution sequence.
tests for shutdown with failIfNotRunning set to true and false
use relative path for derbyHome

the system property "basedir" is not set when launching the tests from
an IDE (Eclipse in my case). Thus, the Derby database is created at
./null/target/derby.
initialize connectionURLShutdown in tests

Due to an uninitialized connectionURLShutdown
DriverManager.getConnection(getConnectionURLShutdown()) failed with the
message "The url cannot be null", producing the test output
  "[ERROR] The url cannot be null"
instead of 
  "[INFO] Derby system shutdown."

The error was compensated by the subsequent call to server.shutdown(),
so that the tests were still successful.