log all mtr output in vardir/log/stdout.log

despite the name, it logs both stdout and stderr

mysql-test/lib/My/Tee.pm

@@ -0,0 +1,23 @@
+package My::Tee;
+
+# see PerlIO::via
+
+our $copyfh;
+
+sub PUSHED
+{
+  open($copyfh, '>', "$::opt_vardir/log/stdout.log")
+    or die "open(>$::opt_vardir/log/stdout.log): $!"
+      unless $copyfh;
+ bless { }, shift;
+}
+
+sub WRITE
+{
+ my ($obj, $buf, $fh) = @_;
+ print $fh $buf;
+ print $copyfh $buf;
+ return length($buf);
+}
+
+1;