Add .log file ending to tap test log files

Hi,

Right now it's harder than necessary to capture the log output from tap
tests because the the regression tests files don't end with a common
file ending with other types of logs. They're
# Open the test log file, whose name depends on the test name.
$test_logfile = basename($0);
$test_logfile =~ s/\.[^.]+$//;
$test_logfile = "$log_path/regress_log_$test_logfile";

This was essentially introduced in 1ea06203b82: "Improve logging of TAP tests."

Would anybody object to replacing _logfile with .log? I realize that'd
potentially would cause some short-term pain on the buildfarm, but I
think it'd improve things longer term.

Greetings,

Andres Freund