Added JUnit Test Cases for FormatFileCallable

[surajdm123]

Changes

This PR adds the missing unit test case for FormatFileCallable.

Test Plan

mvn -Dtest=FormatFileCallableResultTest test

[INFO] -------------------------------------------------------
[INFO]  T E S T S
[INFO] -------------------------------------------------------
[INFO] Running com.google.googlejavaformat.java.FormatFileCallableResultTest
[INFO] Tests run: 5, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.031 s - in com.google.googlejavaformat.java.FormatFileCallableResultTest
[INFO] 
[INFO] Results:
[INFO] 
[INFO] Tests run: 5, Failures: 0, Errors: 0, Skipped: 0
[INFO] 

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mvn test

Next Steps

• Add more missing test cases to make this codebase more resilient.