test(audience-sdk): clean up missed Progression.Level and Gzip placeholder references

Two inline references survived earlier sweeps:
- GzipTests:15 had `WireFixture.Track((MessageFields.EventName, "test"))`;
  the test exercises the gzip round trip and the event name is filler.
  Migrated to TestEventNames.PlaceholderTest, matching the
  HttpTransportTests pattern.
- ImmutableAudienceTests:585 had `Level = "1"` in a Progression Track
  fixture. Migrated to TestFixtures.ProgressionLevelFixture, paired
  with the existing TypedEventTests usage.

Per the user's "everything random goes in a constant" stance.

Follow-up to SDK-272 (centralisation of duplicated literals).

Author: ImmutableJeffrey

src/Packages/Audience/Tests/Runtime/ImmutableAudienceTests.cs

@@ -582,7 +582,7 @@ public void Track_TypedProgression_WritesCorrectEventName()
             {
                 Status = ProgressionStatus.Complete,
                 World = TestFixtures.ProgressionWorldTutorial,
-                Level = "1"
+                Level = TestFixtures.ProgressionLevelFixture
             });
             ImmutableAudience.Shutdown();
 

src/Packages/Audience/Tests/Runtime/Utility/GzipTests.cs

@@ -12,7 +12,7 @@ internal class GzipTests
         [Test]
         public void Compress_ProducesValidGzip_ThatDecompressesToOriginal()
         {
-            var original = WireFixture.Track((MessageFields.EventName, "test"));
+            var original = WireFixture.Track((MessageFields.EventName, TestEventNames.PlaceholderTest));
 
             var compressed = Gzip.Compress(original);