Fix stackoverflow exception in testConnection

[nPraml]

Hello @rbygrave,

During development, we encountered an issue where an exception occurred during container creation, and the connection completion escalated into an infinite loop.
Stacktrace:

	at io.ebean.datasource/io.ebean.datasource.pool.ConnectionPool.returnConnectionForceClose(ConnectionPool.java:545)
	at io.ebean.datasource/io.ebean.datasource.pool.PooledConnection.close(PooledConnection.java:516)
	at io.ebean.datasource/io.ebean.datasource.pool.ConnectionPool.testConnection(ConnectionPool.java:397)
	at io.ebean.datasource/io.ebean.datasource.pool.ConnectionPool.returnTheConnection(ConnectionPool.java:563)
	at io.ebean.datasource/io.ebean.datasource.pool.ConnectionPool.returnConnectionForceClose(ConnectionPool.java:545)
	at io.ebean.datasource/io.ebean.datasource.pool.PooledConnection.close(PooledConnection.java:516)
	at io.ebean.datasource/io.ebean.datasource.pool.ConnectionPool.testConnection(ConnectionPool.java:397)
	at io.ebean.datasource/io.ebean.datasource.pool.ConnectionPool.returnTheConnection(ConnectionPool.java:563)
	at io.ebean.datasource/io.ebean.datasource.pool.ConnectionPool.returnConnectionForceClose(ConnectionPool.java:545)
	at io.ebean.datasource/io.ebean.datasource.pool.PooledConnection.close(PooledConnection.java:516)
	at io.ebean.datasource/io.ebean.datasource.pool.ConnectionPool.testConnection(ConnectionPool.java:397)
	at io.ebean.datasource/io.ebean.datasource.pool.ConnectionPool.returnTheConnection(ConnectionPool.java:563)
	at io.ebean.datasource/io.ebean.datasource.pool.ConnectionPool.returnConnectionForceClose(ConnectionPool.java:545)

We have implemented a proposed fix in this PR.

Can you check it please?