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/upper_case_hex_literals.sol

@@ -0,0 +1,9 @@
+contract test {
+
+    function f() public pure returns (uint256) {
+        uint256 a = 0x1234aAbcC;
+        uint256 b = 0x1234ABCDEF;
+        return a + b;
+    }
+}
+// ----