classfields: add tests for forbidden propertynames

# Conflicts:
#	src/class-fields/propname-constructor.case
#	src/class-fields/propname-error/cls-decl-computed-name.template
#	src/class-fields/propname-error/cls-decl-literal-name.template
#	src/class-fields/propname-error/cls-decl-string-name.template
#	src/class-fields/propname-error/cls-expr-computed-name.template
#	src/class-fields/propname-error/cls-expr-literal-name.template
#	src/class-fields/propname-error/cls-expr-string-name.template

src/class-fields/static-propname-constructor.case

+// Copyright (C) 2017 Valerie Young. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+desc: Static class field cannot have PropName 'constructor'
+info: |
+
+  // This test file tests the following early error:
+  Static Semantics: Early Errors
+
+    ClassElement : staticFieldDefinition;
+      It is a Syntax Error if PropName of FieldDefinition is "prototype" or "constructor".
+features: [class-fields]
+template: propname-error
+---*/
+
+//- static
+static
+//- propname
+constructor

src/class-fields/static-propname-prototype.case

+// Copyright (C) 2017 Valerie Young. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+desc: Static class fields cannot have PropName 'prototype'
+info: |
+
+  // This test file tests the following early error:
+  Static Semantics: Early Errors
+
+    ClassElement : staticFieldDefinition;
+      It is a Syntax Error if PropName of FieldDefinition is "prototype" or "constructor".
+features: [class-fields]
+template: propname-error
+---*/
+
+//- static
+static
+//- propname
+prototype