Fix scene unload error on AfterTest in CreatedSceneAttribute

[nowsprinting]

Problem

Can not unload attribute-created-scene on AfterTest method in CreatedSceneAttribute.

AfterTest : System.ArgumentException : Scene to unload is invalid

Condition

Unload attribute-created-scene in test.
e.g., LoadScene

Fixes

Skip unload if attribute-created-scene was unloaded.

refs #113

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

[github-actions]

Code Metrics Report

	master (a4a5608)	#116 (5b3790a)	+/-
Coverage	78.7%	78.8%	+0.0%
Code to Test Ratio	1:0.7	1:0.7	+0.0
Test Execution Time	3m56s	3m40s	-16s

Details

  |                     | master (a4a5608) | #116 (5b3790a) |  +/-  |
  |---------------------|------------------|----------------|-------|
+ | Coverage            |            78.7% |          78.8% | +0.0% |
  |   Files             |               35 |             35 |     0 |
  |   Lines             |             1064 |           1067 |    +3 |
+ |   Covered           |              838 |            841 |    +3 |
+ | Code to Test Ratio  |            1:0.7 |          1:0.7 |  +0.0 |
  |   Code              |             1859 |           1863 |    +4 |
+ |   Test              |             1448 |           1457 |    +9 |
+ | Test Execution Time |            3m56s |          3m40s |  -16s |

Code coverage of files in pull request scope (100.0% → 100.0%)

Files	Coverage	+/-
Runtime/Attributes/CreateSceneAttribute.cs	100.0%	0.0%

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Reported by octocov