add a test for the coverage map

This also enforces our strict 1:1 mapping from tests to the system under test.

test/map.js

@@ -0,0 +1,35 @@
+const t = require('tap')
+
+// ensure that the coverage map maps all coverage
+const ignore = ['.git', '.github', 'node_modules', 'coverage', 'tap-snapshots', 'test', 'fixtures']
+const {statSync, readdirSync} = require('fs')
+const find = (folder, set = [], root = true) => {
+  const ent = readdirSync(folder)
+  set.push(...ent.filter(f => /\.m?js$/.test(f)).map(f => folder + '/' + f))
+  for (const f of ent.filter(f => !ignore.includes(f) && !/\.m?js$/.test(f))) {
+    if (statSync(folder + '/' + f).isDirectory())
+      find(folder + '/' + f, set, false)
+  }
+  if (!root)
+    return
+  return set.map(f => f.substr(folder.length + 1)
+    .replace(/\\/g, '/'))
+    .sort((a, b) => a.localeCompare(b))
+}
+
+const {resolve} = require('path')
+const root = resolve(__dirname, '..')
+
+const sut = find(root)
+const tests = find(root + '/test')
+t.strictSame(sut, tests, 'test files should match system files')
+const map = require('../map.js')
+
+for (const testFile of tests) {
+  t.test(testFile, t => {
+    t.plan(1)
+    // cast to an array, since map() can return a string or array
+    const systemFiles = [].concat(map(testFile))
+    t.ok(systemFiles.some(sys => sut.includes(sys)), 'test covers a file')
+  })
+}