t/harness: avoid tests outside the src tree

[perl #124050] t/harness can mistakenly run tests outside of the perl
    source tree

cfa5625 made t/harness prepend '../' to test filenames if such a
file existed. This allowed things like

    ./perl t/harness cpan/foo/t/foo.t

to work even after harness had done a chdir("t").  However, it was then
picking up a ../cpan/foo/t/foo.t file outside the src tree in preference
to the one inside.

Add belt-and-brace conditions: only modify the filename if the unmodified
file doesn't exist, and only only if it doesn't already start with ../.

Based on an earlier patch by Matthew Horsfall

Author: iabyn

t/harness

@@ -188,7 +188,7 @@ if ($^O eq 'MSWin32') {
 
 # Allow eg ./perl t/harness t/op/lc.t
 for (@tests) {
-    if (-f "../$_") {
+    if (! -f $_ && !/^\.\./ && -f "../$_") {
         $_ = "../$_";
         s{^\.\./t/}{};
     }