Remove bashism; sh != bash on Ubuntu.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@54118 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/test/TestRunner.sh b/test/TestRunner.sh
index 36cec72..767d966 100755
--- a/test/TestRunner.sh
+++ b/test/TestRunner.sh
@@ -42,7 +42,7 @@
 
 # Run under valgrind if the VG environment variable has been set.
 CLANG=$CLANG
-if [ "$CLANG" == "" ]; then
+if [ ! -n "$CLANG" ]; then
     CLANG="clang"
 fi
 if [ -n "$VG" ]; then