Add two tests for TRAP_* signal codes for SIGTRAP.

• ptrace__breakpoint_siginfo tests that a SIGTRAP for a software breakpoint in userland triggers a SIGTRAP with a signal code of TRAP_BRKPT.
• ptrace__step_siginfo tests that a SIGTRAP reported for a step after stepping via PT_STEP or PT_SETSTEP has a signal code of TRAP_TRACE.