Rename and rearrange entities in tests to avoid name reuse (#7656)

Fix a bunch of cases where we use the same external name to mean
multiple different things in the same test. We've historically gotten
away with this, but under `--share-cpp-ast`, it becomes an error, at
least if the entity is either defined in, or used from, C++ code.

Assisted-by: Gemini via Antigravity (original change) and Claude Code
(suggested edits in review)

---------

Co-authored-by: Chandler Carruth <chandlerc@gmail.com>

toolchain/check/testdata/interop/cpp/class/export/static.carbon

@@ -14,13 +14,13 @@
 library "[[@TEST_NAME]]";
 import Cpp;
 
-class C {
+class PublicClass {
   static var x: i32 = 123;
 }
 
 inline Cpp '''
-int F() {
-  return Carbon::C::x;
+int PublicStatic() {
+  return Carbon::PublicClass::x;
 }
 ''';
 
@@ -28,19 +28,19 @@ int F() {
 library "[[@TEST_NAME]]";
 import Cpp;
 
-class C {
+class PrivateClass {
   private static var x: i32 = 123;
 }
 
 inline Cpp '''
-int F() {
-  // CHECK:STDERR: fail_static_private.carbon:[[@LINE+7]]:21: error: 'x' is a private member of 'Carbon::C' [CppInteropParseError]
-  // CHECK:STDERR:    17 |   return Carbon::C::x;
-  // CHECK:STDERR:       |                     ^
+int PrivateStatic() {
+  // CHECK:STDERR: fail_static_private.carbon:[[@LINE+7]]:32: error: 'x' is a private member of 'Carbon::PrivateClass' [CppInteropParseError]
+  // CHECK:STDERR:    17 |   return Carbon::PrivateClass::x;
+  // CHECK:STDERR:       |                                ^
   // CHECK:STDERR: fail_static_private.carbon:[[@LINE-8]]:23: note: implicitly declared private here [CppInteropParseNote]
   // CHECK:STDERR:     5 |   private static var x: i32 = 123;
   // CHECK:STDERR:       |                       ^
   // CHECK:STDERR:
-  return Carbon::C::x;
+  return Carbon::PrivateClass::x;
 }
 ''';