Set up GTest test suite

[MihaiStreames]

Goal

Add a test suite covering current functionality, split by feature.

Test files

file	covers
tests/test_expected.cpp	Expected<T> value/error paths, Result<T> alias
tests/test_failure.cpp	Failure construction, push(), ring buffer, downcast, chain(), root_cause()
tests/test_macros.cpp	ANYHOW_TRY, ANYHOW_TRY_CATCH, ANYHOW_TRY_ASSIGN, ANYHOW_BAIL, ANYHOW_ENSURE
tests/test_scope_guard.cpp	fire on destroy, release(), move constructor, no double-fire
tests/test_fmt.cpp	fmt(), operator<<, frame display
tests/test_context.cpp	context(), with_context()

Setup

• GTest via CMake FetchContent
• Tests run in CI