Do not use incrementing numbers in test names (#6821)

This makes the test names unstable.  In this case, according to the
commit summary of ab327ac0, the problem is just overloaded operations,
so we should be able to disambiguate in this case by providing the types
of the arguments, which that commit itself did in two other places.

I also separated arguments with ", " instead of just "," to make them
look nicer, in the two existing places as well.
diff --git a/resources/idlharness.js b/resources/idlharness.js
index d12748f..adaef20 100644
--- a/resources/idlharness.js
+++ b/resources/idlharness.js
@@ -1687,7 +1687,7 @@
 {
     var a_test = async_test(this.name + " interface: operation " + member.name +
                             "(" + member.arguments.map(
-                                function(m) {return m.idlType.idlType; } )
+                                function(m) {return m.idlType.idlType; } ).join(", ")
                             +")");
     a_test.step(function()
     {
@@ -2182,6 +2182,11 @@
         || member.type == "operation")
         && member.name)
         {
+            var described_name = member.name;
+            if (member.type == "operation")
+            {
+                described_name += "(" + member.arguments.map(arg => arg.idlType.idlType).join(", ") + ")";
+            }
             test(function()
             {
                 assert_equals(exception, null, "Unexpected exception when evaluating object");
@@ -2221,7 +2226,7 @@
                         assert_equals(typeof obj[member.name], "function");
                     }
                 }
-            }.bind(this), this.name + " interface: " + desc + ' must inherit property "' + member.name + '" with the proper type (' + i + ')');
+            }.bind(this), this.name + " interface: " + desc + ' must inherit property "' + described_name + '" with the proper type');
         }
         // TODO: This is wrong if there are multiple operations with the same
         // identifier.
@@ -2229,7 +2234,7 @@
         if (member.type == "operation" && member.name && member.arguments.length)
         {
             var a_test = async_test( this.name + " interface: calling " + member.name +
-            "(" + member.arguments.map(function(m) { return m.idlType.idlType; }) +
+            "(" + member.arguments.map(function(m) { return m.idlType.idlType; }).join(", ") +
             ") on " + desc + " with too few arguments must throw TypeError");
             a_test.step(function()
             {