[lldb][TerminalTest] Fix assertion failure

[splhack]

Summary

D152712 replaced llvm::sys::RetryAfterSignal(-1, ::open) with
FileSystem::Instance().Open for bionic in PseudoTerminal::OpenSecondary, and
FileSystem::Instance() is failing with assertion on arm Linux.

The assertion should be FileSystem re-initialization check, therefore the
hypothesis is that TerminalTest tests are initializing FileSystem instance
repeatedly.

Use SubsystemRAII<FileSystem> to ensure tearing down the FileSystem instance.

[JDevlieghere]

LGTM

lldb/unittests/Host/posix/TerminalTest.cpp

 //===-- TerminalTest.cpp --------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
+#include "lldb/Host/FileSystem.h"
 #include "lldb/Host/PseudoTerminal.h"
 #include "lldb/Host/Terminal.h"
 #include "llvm/Testing/Support/Error.h"
+#include "TestingSupport/SubsystemRAII.h"
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
 
 #include <termios.h>
 #include <unistd.h>
 
 using namespace lldb_private;
 
 class TerminalTest : public ::testing::Test {
 protected:
+  SubsystemRAII<FileSystem> subsystems;
   PseudoTerminal m_pty;
   int m_fd;
   Terminal m_term;
 
   void SetUp() override {
     ASSERT_THAT_ERROR(m_pty.OpenFirstAvailablePrimary(O_RDWR | O_NOCTTY),
                       llvm::Succeeded());
     ASSERT_THAT_ERROR(m_pty.OpenSecondary(O_RDWR | O_NOCTTY),