CPP: Add c++17 flag to the test.

Author: geoffw0

cpp/ql/test/query-tests/Best Practices/Likely Errors/EmptyBlock/EmptyBlock.expected

@@ -1,3 +1,3 @@
-| empty_block.cpp:7:10:7:11 | { ... } | Empty block without comment |
-| empty_block.cpp:10:10:11:3 | { ... } | Empty block without comment |
-| empty_block.cpp:18:10:19:3 | { ... } | Empty block without comment |
+| empty_block.cpp:9:10:9:11 | { ... } | Empty block without comment |
+| empty_block.cpp:12:10:13:3 | { ... } | Empty block without comment |
+| empty_block.cpp:20:10:21:3 | { ... } | Empty block without comment |

cpp/ql/test/query-tests/Best Practices/Likely Errors/EmptyBlock/empty_block.cpp

@@ -1,3 +1,5 @@
+// semmle-extractor-options: -std=c++17
+
 // GOOD:
 void f() {
 }