Set log_line_prefix and application name in test drivers

Before pg_regress runs psql, set the application name to the test name.
Similarly, set the application name to the test file name in the TAP
tests.  Also, set a default log_line_prefix that show the application
name, as well as the PID and a time stamp.

That way, the server log output can be correlated to the test input
files, making debugging a bit easier.