Fix llvm-cov test as suggested in r196228's post commit review.

llvm-svn: 196255
diff --git a/llvm/test/tools/llvm-cov/llvm-cov.test b/llvm/test/tools/llvm-cov/llvm-cov.test
index 69fed09..0357b21 100644
--- a/llvm/test/tools/llvm-cov/llvm-cov.test
+++ b/llvm/test/tools/llvm-cov/llvm-cov.test
@@ -1,3 +1,5 @@
+REQUIRES: shell
+RUN: rm -f test.*
 RUN: cp %p/Inputs/test.* .
 RUN: llvm-cov -gcno=test.gcno -gcda=test.gcda
 RUN: diff -aub test.cpp.gcov test.cpp.llcov