test: add test-debugger-breakpint-exists

In Node.js 15, calling `setBreakpoint(1)` and `restart` twice in a row
caused the debugger to exit. In Node.js 16, it no longer exits but
throws an error that is expected, or at least reasonable, or at least
better than exiting.

The error message previously had `undefined` appended to it. It no
longer does.

This adds test coverage to `unpackError()` in
`lib/internal/debugger/inspect_client.js`. That function previously was
untested.

Refs: https://github.com/nodejs/node-inspect/issues/101

test/sequential/test-debugger-breakpoint-exists.js

@@ -0,0 +1,31 @@
+'use strict';
+
+const common = require('../common');
+
+common.skipIfInspectorDisabled();
+
+const fixtures = require('../common/fixtures');
+const startCLI = require('../common/debugger');
+
+// Test for "Breakpoint at specified location already exists" error.
+{
+  const script = fixtures.path('debugger', 'three-lines.js');
+  const cli = startCLI([script]);
+
+  function onFatal(error) {
+    cli.quit();
+    throw error;
+  }
+
+  cli.waitForInitialBreak()
+    .then(() => cli.waitForPrompt())
+    .then(() => cli.command('setBreakpoint(1)'))
+    .then(() => cli.command('restart'))
+    .then(() => cli.waitForInitialBreak())
+    .then(() => cli.waitForPrompt())
+    .then(() => cli.command('setBreakpoint(1)'))
+    .then(() => cli.command('restart'))
+    .then(() => cli.waitFor(/Breakpoint at specified location already exists/))
+    .then(() => cli.quit())
+    .then(null, onFatal);
+}