Changing depends to test-depends, since we need it for that

JJ · JJ · commit 1df2cb05d478 · 2020-09-06T10:38:35.000+02:00
This refs #3615, but does not fix it.
diff --git a/META6.json b/META6.json
@@ -7,7 +7,7 @@
     "perl6"
   ],
   "auth": "perl6",
-  "depends": [
+  "test-depends": [
     "File::Temp",
     "Pod::To::HTML:ver<0.6.1+>",
     "Test::META",
