[libTooling][NFC] Switch StencilTest.cpp to use EXPECT_THAT_EXPECTED

Summary:
Currently, some tests use homegrown matchers to handle `llvm::Expected`
values. This revision standardizes on EXPECT_THAT_EXPECTED and `HasValue`.

Reviewers: ilya-biryukov

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67973

llvm-svn: 372918