Refactor repeated code for creating syscalls in raw2trace and invariant checker tests.

Because the syscall instruction differs by the platform, currently the following code are repeated multiple times in raw2trace_unit_tests.cpp and invariant_checker_test.cpp

```
# ifdef X86
instr_t *sys = INSTR_CREATE_syscall(GLOBAL_DCONTEXT);
# elif defined(AARCHXX)
instr_t *sys =
INSTR_CREATE_svc(GLOBAL_DCONTEXT, opnd_create_immed_int((sbyte)0x0, OPSZ_1));
# elif defined(RISCV64)
instr_t *sys = INSTR_CREATE_ecall(GLOBAL_DCONTEXT);
# else
# error Unsupported architecture.
# endif
```

We should refactor this by introducing a macro or helper function.