Remove unnecessary template specifiers from truct constructors in tests.

As far as I can see they are unnecessary and in C++20 they lead to errors.
Removing them the tests pass with C++20 and the default version.
Example error message:
```
llvm-project/llvm/unittests/IR/PassBuilderCallbacksTest.cpp:275:30: error: expected unqualified-id before ‘)’ token
  275 |   MockAnalysisHandle<Module>() { setDefaults(); }
```

Differential Revision: https://reviews.llvm.org/D141358