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.
use plugin version and distinct artifactId in test pom

Using the same maven coordinates for the plugin under test and its
hosting test plugin causes confusion.
Now the current plugin version is injected by resource filtering.
The new host plugin name expresses its dummy purpose.
let Maven preconfigure the Mojos with their default values