Replace StringBuffers to StringBuilders in tests

[Sergei Ustimenko]

Hi all,

Some tests use StringBuffers instead of StringBuilders where additional thread-safety
is not required as e.g. in test/jdk/sun/util/resources/TimeZone/Bug4640234.java:82 :
...

StringBuffer errors

=

new

StringBuffer(

""

);

StringBuffer warnings

=

new

StringBuffer(

""

);

...
There were some efforts to clean up core libs (e.g. java.base module in
https://github.com/openjdk/jdk/pull/2922) and I've noticed some tests that could be
improved as well.

Now there are about 300 tests for different modules that in general use StringBuffers
(most probably some of them not without a reason) so is it something worth looking
into? What you think about it?

Regards,
Sergei