[LLDB][LoongArch] Add unittests for EmulateInstructionLoongArch

Add unit tests For EmulateInstructionLoongArch existing branch instruction.
Add 19 test cases in total.

Without this patch:

```
$ ninja check-lldb-unit
[0/1] Running lldb unit test suite

Testing Time: 10.55s
  Passed: 1025
```

With this patch:

```
$ ninja check-lldb-unit
[0/1] Running lldb unit test suite

Testing Time: 10.45s
  Passed: 1044
```

Reviewed By: DavidSpickett

Differential Revision: https://reviews.llvm.org/D140386