rename test for consistency with other pragma test.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@69539 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/test/Preprocessor/poison.c b/test/Preprocessor/pragma_poison.c
similarity index 100%
rename from test/Preprocessor/poison.c
rename to test/Preprocessor/pragma_poison.c