test: remove duplicated buffer negative allocation test

The buffer negative allocation test test-buffer-negative-length.js is almost duplicated with test-buffer-no-negative-allocation.js.

https://github.com/nodejs/node/blob/3fbf55a87c5c74fef93b16721834f9e270796cef/test/parallel/test-buffer-negative-length.js#L1-L17

https://github.com/nodejs/node/blob/3fbf55a87c5c74fef93b16721834f9e270796cef/test/parallel/test-buffer-no-negative-allocation.js#L1-L28

So remove test-buffer-negative-length.js and add SlowBuffer allocation test into test-buffer-no-negative-allocation.js.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines