lib/test: Fall back to the skip message for test titles

Skips like [1]:

  ok 23 # skip Insufficient flogiston pressure.

are parsed (at least by [email protected]) into structures like:

  {
    id: 1,
    name: "",
    ok: true,
    skip: "root.readonly is false but the root filesystem is still not writable"
  }

But title-less tests are not very useful.  With this change, cases
like the above empty-string name will fall back to the skip value.
And if that skip value is undefined, we'll fall back to an empty
string (because I'm not sure how well downstream consumers would
handle and undefined title).

One positive effect of this change is that Mochawesome now has a title
message to render for these skips (where previously it just used an
empty h4).

[1]: https://testanything.org/tap-version-13-specification.html#skipping-tests

lib/test.js

@@ -6,7 +6,7 @@ function Test (result, suite) {
   this.result = result
   this._slow = 75
   this.duration = result.time
-  this.title = result.name
+  this.title = result.name || result.skip || ''
   this.pending = result.todo || false
   if (result.ok) {
     this.state = result.skip ? 'skipped' : 'passed'