Make unittests pass with "npm run coverage" on Node 14

[smcclure15]

I'm on Node 14.15.0 (on Mac), running against QUnit 2.15.0 source.

npm test runs cleanly, but npm run coverage reveals two failures in the CLI tests:

not ok 19 CLI Main > run ESM test suite with import statement
  ---
  message: "Promise rejected during "run ESM test suite with import statement": Command failed: /bin/sh -c ../../../bin/qunit.js ../../es2018/esm.mjs

TAP version 13
not ok 1 ../../es2018/esm.mjs > Failed to load the test file with error:
/Users/stevemcclure/Documents/GitHub/qunit/test/es2018/esm.mjs:1
import sum from "./sum.mjs";
^^^^^^

SyntaxError: Cannot use import statement outside a module

and

not ok 21 CLI Main > mapped trace with native source map
  ---
  message: "failed"
  severity: failed
  actual  : "TAP version 13\nok 1 Example > good\nnot ok 2 Example > bad\n  ---\n  message: \"failed\"\n  severity: failed\n  actual  : false\n  expected: true\n  stack:     at Assert.pushResult (/qunit/qunit/qunit.js)\n  ...\n1..2\n# pass 1\n# skip 0\n# todo 0\n# fail 1"
  expected: "TAP version 13\nok 1 Example > good\nnot ok 2 Example > bad\n  ---\n  message: \"failed\"\n  severity: failed\n  actual  : false\n  expected: true\n  stack:     at Object.<anonymous> (/qunit/test/cli/fixtures/sourcemap/sourcemap/source.js:7:10)\n  ...\n1..2\n# pass 1\n# skip 0\n# todo 0\n# fail 1"
  stack:     at Object.<anonymous> (/Users/stevemcclure/Documents/GitHub/qunit/test/cli/main.js:175:12)

Still not sure if it's just a test fix (stdout formatting could be different), or there's any latent bugs here.

[Krinkle]

Disabling the sourcemap test under coverage mode makes sense indeed. LGTM. I hadn't faced it yet since we use Node 10 LTS for the version-agnostic coverage run in CI, and me locally as well.

The syntax error is an interesting one. It seems that may've changed in upstream Node.js 14 within a minor version, due to changes in V8? In any event, I can reproduce it and confirm that Node's own upstream tests check for this the same way, so LGTM on both accounts.