Fix vcregress.pl PROVE_FLAGS bug in commit 93b7d973

This change didn't adjust the publicly visible taptest function, causing
buildfarm failures on bowerbird.

Backpatch to 9.4 like previous change.

src/tools/msvc/vcregress.pl

@@ -227,11 +227,20 @@ sub bincheck
 sub taptest
 {
 	my $dir = shift;
+	my @args;
+
+	if ($dir =~ /^PROVE_FLAGS=/)
+	{
+		push(@args, $dir);
+		$dir = shift;
+	}
 
 	die "no tests found!" unless -d "$topdir/$dir/t";
 
+	push(@args,"$topdir/$dir);
+
 	InstallTemp();
-	my $status = tap_check("$topdir/$dir");
+	my $status = tap_check(@args);
 	exit $status if $status;
 }