[Flang][Runtime][tests] Escape regex special character.

The second argument of `ASSERT_DEATH` describes a regular expression, in
which parentheses have special meaning. Matches of literal parentheses
need to be escaped.

Fixes failure of InvalidFormatFailure.ParenMismatch and
InvalidFormatFailure.ParenMismatch when gtest is compiled with MSVC's
regex implementation.

Reviewed By: awarzynski

Differential Revision: https://reviews.llvm.org/D104011