[lldb][test] Remove symlink for API tests.

Summary: Moves lldbsuite tests to lldb/test/API.

This is a largely mechanical change, moved with the following steps:

```
rm lldb/test/API/testcases
mkdir -p lldb/test/API/{test_runner/test,tools/lldb-{server,vscode}}
mv lldb/packages/Python/lldbsuite/test/test_runner/test lldb/test/API/test_runner
for d in $(find lldb/packages/Python/lldbsuite/test/* -maxdepth 0 -type d | egrep -v "make|plugins|test_runner|tools"); do mv $d lldb/test/API; done
for d in $(find lldb/packages/Python/lldbsuite/test/tools/lldb-vscode -maxdepth 1 -mindepth 1 | grep -v ".py"); do mv $d lldb/test/API/tools/lldb-vscode; done
for d in $(find lldb/packages/Python/lldbsuite/test/tools/lldb-server -maxdepth 1 -mindepth 1 | egrep -v "gdbremote_testcase.py|lldbgdbserverutils.py|socket_packet_pump.py"); do mv $d lldb/test/API/tools/lldb-server; done
```

lldb/packages/Python/lldbsuite/__init__.py and lldb/test/API/lit.cfg.py were also updated with the new directory structure.

Reviewers: labath, JDevlieghere

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D71151

lldb/test/API/functionalities/plugins/command_plugin/plugin.cpp.template

@@ -0,0 +1,53 @@
+//===-- plugin.cpp -------------------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+/*
+An example plugin for LLDB that provides a new foo command with a child subcommand
+Compile this into a dylib foo.dylib and load by placing in appropriate locations on disk or
+by typing plugin load foo.dylib at the LLDB command line
+*/
+
+%include_SB_APIs%
+
+namespace lldb {
+    bool
+    PluginInitialize (lldb::SBDebugger debugger);
+}
+
+class ChildCommand : public lldb::SBCommandPluginInterface
+{
+public:
+    virtual bool
+    DoExecute (lldb::SBDebugger debugger,
+               char** command,
+               lldb::SBCommandReturnObject &result)
+    {
+        if (command)
+        {
+            const char* arg = *command;
+            while (arg)
+            {
+                result.Printf("%s ",arg);
+                arg = *(++command);
+            }
+            result.Printf("\n");
+            return true;
+        }
+        return false;
+    }
+    
+};
+
+bool
+lldb::PluginInitialize (lldb::SBDebugger debugger)
+{
+    lldb::SBCommandInterpreter interpreter = debugger.GetCommandInterpreter();
+    lldb::SBCommand foo = interpreter.AddMultiwordCommand("plugin_loaded_command",NULL);
+    foo.AddCommand("child",new ChildCommand(),"a child of plugin_loaded_command");
+    return true;
+}