fix test_platform usages (pantsbuild#9567)

[ci skip-rust-tests]
[ci skip-jvm-tests]

### Problem

When I pushed runtime_platform into a mixin, I broke some assumptions that the test_platform supporting code was making. 

### Solution

This fixes it and adds a test to make sure it won't break until it's removed in 1.28.

### Result

Using test_platform continues to work.

src/python/pants/backend/jvm/targets/junit_tests.py

@@ -88,8 +88,8 @@ def __init__(
             raise TargetDefinitionException(
                 self, "Cannot specify runtime_platform and test_platform together."
             )
-        if "test_platform" in kwargs and "runtime_platform" not in kwargs:
-            kwargs["runtime_platform"] = kwargs["test_platform"]
+        if "test_platform" in kwargs and not runtime_platform:
+            runtime_platform = kwargs["test_platform"]
             del kwargs["test_platform"]
 
         payload.add_fields(

tests/python/pants_test/backend/jvm/targets/test_junit_tests.py

@@ -88,6 +88,11 @@ def test_implicit_junit_dep(self):
             sorted(str(x) for x in test2.dependencies),
         )
 
+    def test_test_platform_sets_runtime_platform(self):
+        init_subsystem(JUnit)
+        target = self.make_target("//:test1", JUnitTests, sources=[], test_platform="b")
+        assert target.payload.runtime_platform == "b"
+
     def test_runtime_and_test_platform_error(self):
         with self.assertRaisesWithMessage(
             TargetDefinitionException,