Make syntax tree test print the line number when it fails

Summary:
The syntax tree test uses a helper function that executes all testing
assertions. When an assertion fails, the only line number that gets
printed to the log refers to the helper function. After this change, we
would also get the line number of the EXPECT_TRUE macro invocation
(unfortunately, the line number of the last token of it, not the first
one, but there's not much I can do about it).

Reviewers: hlopko, eduucaldas

Reviewed By: hlopko, eduucaldas

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D81107