Wrap check for URI::Escape in BEGIN block

The "unless try" was a good solution, however the check for URI::Escape
needs to happen at compile time, since the later "use" will be checked and
thus fail since URI::Escape isn't there.  What a better solution is, I don't
know, however the test works again now.

t/pod-htmlify.t

@@ -2,10 +2,12 @@ use v6;
 use Test;
 use lib 'lib';
 
-unless (try require URI::Escape) {
-    warn "URI::Escape required to run these tests";
-    plan 0;
-    exit;
+BEGIN {
+    unless (try require URI::Escape) {
+        warn "URI::Escape required to run these tests";
+        plan 0;
+        exit;
+    }
 }
 
 plan 3;