Explicitly casting the integer to lzma_check silences the warning.
Since such an invalid value is needed in multiple tests, a constant
INVALID_LZMA_CHECK_ID was added to tests.h.
The use of 0x1000 for lzma_block.check wasn't optimal as if
the underlying type is a char then 0x1000 will be truncated to 0.
However, in these test cases the value is ignored, thus even with
such truncation the test would have passed.
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