clang-format: remove tests that assume no config file will be found as this is not always the case

These tests fail for developers who place their build directories under the
llvm root directory because llvm's own .clang-format file will be found.
Anyway these cases are covered by FormatStyle.GetStyleOfFile tests
(FormatTest.cpp).

Differential Revision: https://reviews.llvm.org/D28983

llvm-svn: 292787