Add new tests.

test/libsolidity/syntaxTests/abstract/abstract_abstract.sol

@@ -0,0 +1,3 @@
+abstract abstract contract A { constructor() public {} }
+// ----
+// ParserError: (9-17): Contract expected.

test/libsolidity/syntaxTests/abstract/contract.sol

@@ -0,0 +1 @@
+abstract contract A { constructor() public {} }

test/libsolidity/syntaxTests/abstract/interface.sol

@@ -0,0 +1,3 @@
+abstract interface A { }
+// ----
+// ParserError: (0-8): Only contracts can be abstract.

test/libsolidity/syntaxTests/abstract/library.sol

@@ -0,0 +1,3 @@
+abstract library A { }
+// ----
+// ParserError: (0-8): Only contracts can be abstract.

test/libsolidity/syntaxTests/abstract/unimplemented_functions.sol

@@ -0,0 +1,5 @@
+contract A {
+    function a() public;
+}
+// ----
+// TypeError: (0-39): Contract "A" should be marked as abstract.

test/libsolidity/syntaxTests/abstract/unimplemented_functions_inherited.sol

@@ -0,0 +1,7 @@
+abstract contract A {
+    function a() public;
+}
+contract B is A {
+}
+// ----
+// TypeError: (49-68): Contract "B" should be marked as abstract.