UnionInputStreamTest: Suppress ErrorProne warning about missing synchronized

UnionInputStreamTest: Suppress ErrorProne warning about missing synchronized

Error Prone reports that the unsynchronized method skip overrides the
synchronized method in ByteArrayInputStream [1].

This is a test class, so we can just suppress the warning as recommended
in [1].

Note that the suppression causes a warning in Eclipse, because it doesn't
recognize the "UnsynchronizedOverridesSynchronized" as a valid value for
the @SuppressWarnings annotation [2].

[1] https://errorprone.info/bugpattern/UnsynchronizedOverridesSynchronized
[2] https://bugs.eclipse.org/bugs/show_bug.cgi?id=392045

Change-Id: I3e798b448211f1363729091f72fb0ef6a873e599
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>