Rename dashes in a few test files to underscores (#2170)

This is somewhat https://github.com/carbon-language/carbon-lang/blob/trunk/docs/project/cpp_style_guide.md#file-names, but in general, underscores are more consistent.

explorer/testdata/addr/nested_method.carbon

@@ -0,0 +1,37 @@
+
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// RUN: %{explorer} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{explorer} %s
+// CHECK: result: 0
+
+package ExplorerTest api;
+
+class B {
+  var x: i32;
+
+  fn GetSetX[addr me: Self*](x: i32) -> i32 {
+    var oldX: auto = (*me).x;
+    (*me).x = x;
+    return oldX;
+  }
+}
+
+class A {
+  var b: B;
+}
+
+fn Main() -> i32 {
+  var b: B = {.x = 0};
+  var a: A = {.b = b};
+  var x: auto = a.b.GetSetX(42);
+  if (a.b.x == 42) {
+    return x;
+  }
+  return 1;
+}