test: reduce buffer size in buffer-creation test
As the test test/sequential/test-buffer-creation-regression.js fails in SmartOS, it would be better not to allocate a huge chunk of memory.
Ref: https://github.com/nodejs/node/issues/10166
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
commit message follows commit guidelines
Affected core subsystem(s)
test