Add XCTestPlan file extension to `FileType` #605

Job	Run time
Test dependencies resolver (macos-15, 16.1)	2m 31s
Test dependencies resolver (macos-14, 15.4)	2m 15s
Test on macOS (macos-14, 15.4)	1m 39s
Test on macOS (macos-13, 14.3.1)	2m 42s
Test on Linux (5.10)	1m 25s
Test without SPM on macOS (macos-13, 14.3.1)	2m 58s
Test on Linux (6.0)	1m 31s
Test on macOS (macos-15, 16.1)	1m 36s
Test without SPM on macOS (macos-15, 16.1)	2m 18s
Test without SPM on macOS (macos-14, 15.4)	2m 11s
Test without SPM on Linux (5.9)	1m 58s
Test on Linux (5.9)	1m 15s
Test without SPM on Linux (5.10)	5m 23s
Test without SPM on Linux (6.0)	2m 59s
	32m 41s

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