test: increase _debugger coverage

The uncaught exception test for `_debugger.js` was not exercising some
code (particularly concerning `interface_.child`) because of the
synchronous nature of the test. This adds an asynchronous version to
increase test coverage.

PR-URL: #8403
Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/fixtures/debug-uncaught-async.js

@@ -0,0 +1,18 @@
+'use strict';
+
+require('../common');
+const assert = require('assert');
+const debug = require('_debugger');
+
+function emit() {
+  const error = new Error('fhqwhgads');
+  process.emit('uncaughtException', error);
+}
+
+assert.doesNotThrow(emit);
+
+// Send debug.start() an argv array of length 1 to avoid code that exits 
+// if argv is empty.
+debug.start(['sterrance']);
+
+setImmediate(emit);

test/parallel/test-debug-uncaught-exception-async.js

@@ -0,0 +1,22 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const path = require('path');
+const spawn = require('child_process').spawn;
+
+const emitUncaught = path.join(common.fixturesDir, 'debug-uncaught-async.js');
+const result = spawn(process.execPath, [emitUncaught], {encoding: 'utf8'});
+
+var stderr = '';
+result.stderr.on('data', (data) => {
+  stderr += data;
+});
+
+result.on('close', (code) => {
+  const expectedMessage =
+    "There was an internal error in Node's debugger. Please report this bug.";
+
+  assert.strictEqual(code, 1);
+  assert(stderr.includes(expectedMessage));
+  assert(stderr.includes('Error: fhqwhgads'));
+});