[clang] [clang-repl] Set up executor implicitly to account for init PTUs (PR #84758)

[Stefan Gränitz via cfe-commits]

weliveindetail wrote:

Oh indeed, it seems we can decide to skip tests in `SetUp()`

```
class FooTest : public ::testing:Test {
protected:
    void SetUp()
    {
        GTEST_SKIP();
    }
};
```

https://github.com/llvm/llvm-project/pull/84758