Remove redundant anchor in preprocess expression

Expr patterns are anchored to the beginning by default. Specifying
the carrot is undefined behavior and generates warnings on some versions, obscuring the
output.

libexec/bats-preprocess

@@ -34,7 +34,7 @@ index=0
 
 while IFS= read -r line; do
   index=$(($index + 1))
-  quoted_name="$(expr "$line" : '^ *@test  *\([^ ].*\)  *{ *$' || true)"
+  quoted_name="$(expr "$line" : ' *@test  *\([^ ].*\)  *{ *$' || true)"
 
   if [ -n "$quoted_name" ]; then
     name="$(eval echo "$quoted_name")"