[lldb] [gdb-remote] Support QEnvironment fallback to hex-encoded

Fall back to QEnvironmentHexEncoded if QEnvironment is not supported.
The latter packet is an LLDB extension, while the former is universally
supported.

Add tests for both QEnvironment and QEnvironmentHexEncoded packets,
including both use due to characters that need escaping and fallback
when QEnvironment is not supported.

Differential Revision: https://reviews.llvm.org/D108018