[libunwind] Mark test as requiring Unix headers (#222679)
Test `libunwind/test/dwarf_expression_stack.pass.cpp` requires a POSIX
environment because it uses `signal.h` and associated functions. Because
of this, the test fails in baremetal targets.
This change adjusts the requirements of the test. `has-unix-headers`
does not map exactly to the requirements here, but it seems to be enough
for this case.
