Check that functions exist in isoltest.

test/libsolidity/semanticTests/empty_contract.sol

@@ -2,5 +2,6 @@ contract test {
 }
 // ====
 // compileViaYul: also
+// allowNonExistingFunctions: true
 // ----
 // i_am_not_there() -> FAILURE

test/libsolidity/semanticTests/functionCall/member_accessors.sol

@@ -13,6 +13,7 @@ contract test {
     uint256 super_secret_data;
 }
 // ====
+// allowNonExistingFunctions: true
 // compileViaYul: also
 // ----
 // data() -> 8

test/libsolidity/semanticTests/functionCall/multiple_functions.sol

@@ -4,6 +4,8 @@ contract test {
     function c() public returns(uint n) { return 2; }
     function f() public returns(uint n) { return 3; }
 }
+// ====
+// allowNonExistingFunctions: true
 // ----
 // a() -> 0
 // b() -> 1

test/libsolidity/semanticTests/smoke/failure.sol

@@ -14,6 +14,7 @@ contract C {
     }
 }
 // ====
+// allowNonExistingFunctions: true
 // EVMVersion: >homestead
 // ----
 // _() -> FAILURE

test/libsolidity/semanticTests/smoke/multiline.sol

@@ -4,6 +4,7 @@ contract C {
     }
 }
 // ====
+// allowNonExistingFunctions: true
 // compileViaYul: also
 // ----
 // f(uint256,uint256,uint256,uint256,uint256): 1, 1, 1, 1, 1

test/libsolidity/semanticTests/viaYul/smoke_test.sol

@@ -1,6 +1,7 @@
 contract C {
 }
 // ====
+// allowNonExistingFunctions: true
 // compileViaYul: true
 // ----
 // f() -> FAILURE