[FLINK-35673][test] Correct the SELF type of TestFileSourceBuilder #54

Job	Run time
Default (Java 8) / Compile	27m 46s
Pre-compile Checks / Basic QA	4m 52s
Default (Java 8) / Test packaging/licensing	31m 29s
Default (Java 8) / E2E (group 1)	2h 30m 16s
Default (Java 8) / Test (module: core)	34m 41s
Default (Java 8) / E2E (group 2)	2h 32m 18s
Default (Java 8) / Test (module: python)	31m 18s
Default (Java 8) / Test (module: table)	35m 27s
Default (Java 8) / Test (module: connect)	18m 42s
Default (Java 8) / Test (module: tests)	46m 20s
Default (Java 8) / Test (module: misc)	45m 0s
	9h 38m 9s

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