[java] fix missing dependency check in the Maven tests

[jeremydubreil]

The variable SOURCES was not defined, which means that changing the Java sources would not trigger the test if those had already been run before.

Without these changes, one could run:

$ make build_mvn_test

change the Java source file in a way that would change the list of Infer warnings, then, run again:

make build_mvn_test

and the test would still pass. This pull request fixes this.

All the tests are passing after the changes using make -j test.

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[jvillard]

Thank you!