Hardcode the integers instead of converting the ZLibNative values, as…

… the Brotli test project also inherits and uses this test.

src/libraries/Common/tests/System/IO/Compression/CompressionStreamUnitTestBase.cs

@@ -513,13 +513,10 @@ async Task<long> GetLengthAsync(int compressionLevel)
                 return mms.Length;
             }
 
-            // Avoid comparing compression level numbers that are too close to each other, as for those cases the expected file size difference cannot be guaranteed
-            long noCompressionLength = await GetLengthAsync((int)ZLibNative.CompressionLevel.NoCompression);
-            long fastestLength = await GetLengthAsync((int)ZLibNative.CompressionLevel.BestSpeed);
-            long optimalLength = await GetLengthAsync((int)ZLibNative.CompressionLevel.DefaultCompression);
-            long smallestLength = await GetLengthAsync((int)ZLibNative.CompressionLevel.BestCompression);
+            long fastestLength = await GetLengthAsync(1);
+            long optimalLength = await GetLengthAsync(5);
+            long smallestLength = await GetLengthAsync(9);
 
-            Assert.True(noCompressionLength >= fastestLength);
             Assert.True(fastestLength >= optimalLength);
             Assert.True(optimalLength >= smallestLength);
         }