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ARROW-13812: [C++] Fix Valgrind error in Grouper.BooleanKey test

Essentially, this failure boils down to: when generating the array of uniques for booleans, we pack 8 bytes at a time into one byte. The bytes are packed from what turns out to be a scratch array allocated by TempVectorStack, which does not initialize its memory. So when we have a non-multiple-of-8 number of bytes, we may end up packing initialized bytes and uninitialized bytes together into a single garbage byte, resulting in Valgrind complaining.

Closes #11041 from lidavidm/arrow-13812

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: Yibo Cai <yibo.cai@arm.com>