tests: Avoid variable shadowing in ASSERT_SIGNAL

tests: Avoid variable shadowing in ASSERT_SIGNAL

The ASSERT_SIGNAL macro uses a fixed variable name, `_r`. This prevents
nesting the macro (like ASSERT_SIGNAL(ASSERT_SIGNAL(...))), as the inner
instance would shadow the outer instance's variable.

Switch to using the UNIQ_T helper to generate unique variable names at
each expansion level. This allows the macro to be used recursively,
which is required for upcoming regression tests regarding signal
handling logic.