test: add test-debugger-breakpoint-exists

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

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

@@ -0,0 +1,27 @@
+'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('setBreakpoint(1)'))
+    .then(() => cli.waitFor(/Breakpoint at specified location already exists/))
+    .then(() => cli.quit())
+    .then(null, onFatal);
+}