Prior to this commit, two tests for exception handling regarding
@DateTimeFormat processing only passed on Java 8. This is due to the
fact that the toString() implementation for annotations changed in Java
9. Specifically, the representation for arrays changed from [] to {},
and strings are enclosed in double quotes beginning with Java 9.

This commit ensures that the affected @DateTimeFormat tests pass on
Java 9+, by making the assertions more lenient regarding toString()
output for annotations.

See gh-26777
See gh-26804