package.json: exclude test from c8 coverage

I had thought that c8 used the same default excludes as nyc, which
includes test{,s}/** but apparently it does not, because test appears in
my coverage results.  Exclude it.

Author: kevinoid

package.json

@@ -90,5 +90,10 @@
   "mocha": {
     "checkLeaks": true,
     "exit": false
+  },
+  "c8": {
+    "exclude": [
+      "test"
+    ]
   }
 }