Add syntax tests to augment the test coverage of

libsolidity/parsing/Scanner.cpp and libsolidity/parsing/Scanner.h
Fix #4627 and PR #5003.

- Add multiline comment test
- Add upper case hex literal test
- Add test for unicode escapes
- Add test for strings with escaped newlines
- Add test for string escapes
- Add test for strings that do not terminate before end of file
- Add test for unterminated blocks

test/libsolidity/syntaxTests/string/string_unterminated_no_new_line.sol

@@ -0,0 +1,4 @@
+contract test {
+    function f() pure public { "abc\
+// ----
+// ParserError: (47-53): Expected primary expression.