[TEST] TreeTest.cpp - Add a comma to avoid build error with -werror

Summary:
The macro `INSTANTIATE_TEST_CASE_P` is defined as
```
\# define INSTANTIATE_TEST_CASE_P(prefix, test_case_name, generator, ...) \
...
```

If we build the test case with -werror, we will get an error like
```
error: ISO C++11 requires at least one argument for the "..." in a
variadic macro

testing::ValuesIn(TestClangConfig::allConfigs()));
                                                ^
```
This patch fixes that.

Reviewers: gribozavr, hlopko, eduucaldas, gribozavr2

Reviewed By: gribozavr2

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D81388