Add .log file ending to tap test log files

On 2021-Mar-04, Andres Freund wrote:

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."

You're misreading this code (I did too): there's no "_logfile" suffix --
$test_logfile is the name of a single variable, it's not $test followed
by _logfile. So the name is "regress_log_001_FOOBAR" with the basename
at the end. But I agree:

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.

Let's add a .log prefix. And also, I would propose a more extensive
renaming, if we're going to do it -- I dislike that the server log files
start with "00x" and the regress ones have the 00x bit in the middle of
the name. So how about we make this
$log_path/$test_logfile.regress.log.

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