Improve and fix some issues in the TAP tests of pg_upgrade

This is based on a set of suggestions from Noah, with the following
changes made:
- The set of databases created in the tests are now prefixed with
"regression" to not trigger any warnings with name restrictions when
compiling the code with -DENFORCE_REGRESSION_TEST_NAME_RESTRICTIONS, and
now only the first name checks after the Windows case of double quotes
mixed with backslashes.
- Fix an issue with EXTRA_REGRESS_OPTS, which were not processed in a
way consistent with 027_stream_regress.pl (missing space between the
option string and pg_regress).  This got introduced in 7dd3ee5.
- Add a check on the exit code of the pg_regress command, to catch
failures after running the regression tests.

Reviewed-by: Noah Misch
Discussion: https://postgr.es/m/YoHhWD5vQzb2mmiF@paquier.xyz

src/bin/pg_upgrade/t/002_pg_upgrade.pl

@@ -13,18 +13,16 @@ use Test::More;
 # Generate a database with a name made of a range of ASCII characters.
 sub generate_db
 {
-	my ($node, $from_char, $to_char) = @_;
+	my ($node, $prefix, $from_char, $to_char, $suffix) = @_;
 
-	my $dbname = '';
+	my $dbname = $prefix;
 	for my $i ($from_char .. $to_char)
 	{
 		next if $i == 7 || $i == 10 || $i == 13;    # skip BEL, LF, and CR
 		$dbname = $dbname . sprintf('%c', $i);
 	}
 
-	# Exercise backslashes adjacent to double quotes, a Windows special
-	# case.
-	$dbname = '\\"\\' . $dbname . '\\\\"\\\\\\';
+	$dbname .= $suffix;
 	$node->command_ok([ 'createdb', $dbname ]);
 }
 
@@ -79,10 +77,12 @@ else
 {
 	# Default is to use pg_regress to set up the old instance.
 
-	# Create databases with names covering most ASCII bytes
-	generate_db($oldnode, 1,  45);
-	generate_db($oldnode, 46, 90);
-	generate_db($oldnode, 91, 127);
+	# Create databases with names covering most ASCII bytes.  The
+	# first name exercises backslashes adjacent to double quotes, a
+	# Windows special case.
+	generate_db($oldnode, 'regression\\"\\', 1,  45,  '\\\\"\\\\\\');
+	generate_db($oldnode, 'regression',      46, 90,  '');
+	generate_db($oldnode, 'regression',      91, 127, '');
 
 	# Grab any regression options that may be passed down by caller.
 	my $extra_opts = $ENV{EXTRA_REGRESS_OPTS} || "";
@@ -99,7 +99,7 @@ else
 
 	my $rc =
 	  system($ENV{PG_REGRESS}
-		  . "$extra_opts "
+		  . " $extra_opts "
 		  . "--dlpath=\"$dlpath\" "
 		  . "--bindir= "
 		  . "--host="
@@ -121,6 +121,7 @@ else
 			print "=== EOF ===\n";
 		}
 	}
+	is($rc, 0, 'regression tests pass');
 }
 
 # Before dumping, get rid of objects not existing or not supported in later