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/multiline_comments.sol

@@ -0,0 +1,13 @@
+/*
+ * This is a multi-line comment
+ * it should create no problems
+ *
+*/
+
+contract test {
+    /*
+    * this is another multi-line comment
+    *
+    */
+}
+// ----