This assertion in TestLegalNotices only records the test case result for passing test cases. It should have a corresponding call to JavadocTester#failed if the case fails:

https://github.com/openjdk/jdk/blob/df599dbb9b0f0ee96d1ec767ac8821f164ab075d/test/langtools/jdk/javadoc/doclet/testLegalNotices/TestLegalNotices.java#L112-L114

Without the call to failed, the test case output looks like:

Starting subtest 1.5: Checking legal notice files
Expected: []
   Found: [ADDITIONAL_LICENSE_INFO, ASSEMBLY_EXCEPTION, LICENSE, jquery.md, jqueryUI.md]

And the failure isn't indicated until the end of the test log:

java.lang.Error: 4 of 58 subtests failed, in 8 runs of javadoc

With the missing call to 'failed', the test case failure is more obvious:

Starting subtest 1.5: Checking legal notice files
Expected: []
   Found: [ADDITIONAL_LICENSE_INFO, ASSEMBLY_EXCEPTION, LICENSE, jquery.md, jqueryUI.md]
FAILED: Did not find all expected files
        at TestLegalNotices.test(TestLegalNotices.java:115)
        at TestLegalNotices.testCombo(TestLegalNotices.java:74)
        at javadoc.tester.JavadocTester.runTest(JavadocTester.java:447)