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ARROW-13042: [C++] Check that kernel output is fully initialized

Enhance TestInitialized() so that it really triggers Valgrind for every uninitialized bit in a buffer's data, including child and dictionary data.

Call TestInitialized() automatically from kernel tests.

Fix the BufferBuilder and TypedBufferBuilder API to really resize a Buffer to 0 when intended; introduce a FinishWithLength() method for cases where the caller wants to force a particular size.

Other fixes to get all tests to pass under Valgrind.

Closes #10550 from pitrou/ARROW-13042-check-kernel-output-initialized

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>