cmd/compile/internal/syntax: added test cases for recent fixes

R=go1.11.

Now that we have a syntax error test harness, we can add the
proper tests for the recent parser fixes.

For #20800.
For #20789.
For #23385.
For #23434.

A test for #20789 already exists in test/fixedbugs, but this
is the better location for that test. But leaving the existing
one where it is as well.

Change-Id: I5937b9b63bafd1efab467a00344302e717976171
Reviewed-on: https://go-review.googlesource.com/88336Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/syntax/testdata/issue20789.src

+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Make sure this doesn't crash the compiler.
+// Line 9 must end in EOF for this test (no newline).
+
+package e
+func([<-chan<-[func /* ERROR unexpected u */ u){go /* ERROR must be function call */
\ No newline at end of file

src/cmd/compile/internal/syntax/testdata/issue23385.src

+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Check error message for use of = instead of == .
+
+package p
+
+func f() {
+	if true || 0 = 1 /* ERROR assignment .* used as value */ {
+	}
+}

src/cmd/compile/internal/syntax/testdata/issue23434.src

+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test case for issue 23434: Better synchronization of
+// parser after missing type. There should be exactly
+// one error each time, with now follow errors.
+
+package p
+
+type T /* ERROR unexpected newline */
+
+type Map map[int] /* ERROR unexpected newline */
+
+// Examples from #23434:
+
+func g() {
+	m := make(map[string] /* ERROR unexpected ! */ !)
+	for {
+		x := 1
+		print(x)
+	}
+}
+
+func f() {
+	m := make(map[string] /* ERROR unexpected \) */ )
+	for {
+		x := 1
+		print(x)
+	}
+}