[lldb][TerminalTest] Fix assertion failure

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.

Differential Revision: https://reviews.llvm.org/D153091