test: increase coverage for Histogram

1. custom inspect test case
Refs: https://coverage.nodejs.org/coverage-18e4f405b14b26f9/lib/internal/histogram.js.html#L56

2. tests that RecordableHistogram is impossible to construct manually
Refs: https://coverage.nodejs.org/coverage-18e4f405b14b26f9/lib/internal/histogram.js.html#L132

PR-URL: #38555
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>

test/parallel/test-perf-hooks-histogram.js

@@ -6,6 +6,7 @@ const {
   createHistogram,
   monitorEventLoopDelay,
 } = require('perf_hooks');
+const { inspect } = require('util');
 
 {
   const h = createHistogram();
@@ -67,3 +68,18 @@ const {
   });
   setTimeout(() => mc.port2.postMessage(e), 100);
 }
+
+{
+  const h = createHistogram();
+  assert(inspect(h, { depth: null }).startsWith('Histogram'));
+  assert.strictEqual(inspect(h, { depth: -1 }), '[RecordableHistogram]');
+}
+
+{
+  // Tests that RecordableHistogram is impossible to construct manually
+  const h = createHistogram();
+  assert.throws(
+    () => new h.constructor(),
+    /^TypeError: illegal constructor$/
+  );
+}