@Test(expected = ComparisonFailure.class)
  public void testCompareStrings() {
    assertEquals("1aaaaaaaa2", "2aaaaaaaa1");
  }
Conclusions:
1. JUnit's string diffing is pretty simplistic (in the test below,
JUnit doesn't find any similarities between the two strings)
2. As soon as at least one string contains a line break,
IDEA offers to compare strings in diff dialog.
spock-junit4/src/test/groovy/org/spockframework/junit/StringComparison.java