[Support] Fix an invalid character escaping in string literal (unittest).

Summary:
A character within a string literal is not escaped correctly.
In this case, there is no semantic change because the invalid character turn out to be NUL anyway.

note: "\0x12" is equivalent to {0, 'x', '1', '2'} and not { 12 }.

This issue was found by clang-tidy.

Reviewers: rnk

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D18747

llvm-svn: 265376