test: upgrade tap, configure coverage thresholds

As part of simplifying the CI setup I plan to not explicitly specify
`--jobs=4` to `tap` going forward. Upgrading to a newer version of `tap`
means that it will automatically parallelize jobs according to available
CPU count, which should be better anyway. Only one of our tests was
incompatible with newer versions of `tap`, so this commit includes a
compatibility fix there.

Also, by default newer versions of `tap` calculate coverage and consider
the test run to fail if below the configured thresholds. The default is
100% coverage and we're not there, so I adjusted the thresholds to match
where we're at for now. We can ratchet those up over time.

test/unit/util_new-block-ids.js

@@ -6,12 +6,11 @@ const test = tap.test;
 let originals;
 let newBlocks;
 
-tap.beforeEach(done => {
+tap.beforeEach(() => {
     originals = simpleStack;
     // Will be mutated so make a copy first
     newBlocks = JSON.parse(JSON.stringify(simpleStack));
     newBlockIds(newBlocks);
-    done();
 });