test: Fix some tests so they fail on expected error, not on syntax error. (#779)

test/language/constructor/cannot_be_infix.wren

@@ -1,5 +1,5 @@
 class Foo {
-  this +(value) { // expect error
+  construct +(value) { // expect error
     System.print("ok")
   }
 }

test/language/constructor/cannot_be_minus.wren

@@ -1,5 +1,5 @@
 class Foo {
-  this -(value) { // expect error
+  construct -(value) { // expect error
     System.print("ok")
   }
 }

test/language/constructor/cannot_be_setter.wren

@@ -1,5 +1,5 @@
 class Foo {
-  this name=(value) { // expect error
+  construct name=(value) { // expect error
     System.print("ok")
   }
 }

test/language/constructor/cannot_be_subscript.wren

@@ -1,5 +1,5 @@
 class Foo {
-  this [value] { // expect error
+  construct [value] { // expect error
     System.print("ok")
   }
 }

test/language/constructor/cannot_be_unary.wren

@@ -1,5 +1,5 @@
 class Foo {
-  this ! { // expect error
+  construct ! { // expect error
     System.print("ok")
   }
 }

test/language/constructor/no_parameter_list.wren

@@ -1,5 +1,5 @@
 class Foo {
-  this new { // expect error
+  construct new { // expect error
     System.print("ok")
   }
 }