Fix bitmask_tests.cpp host accessing device memory

[davidwendt]

This fixes an error in the cleanEndWord utility in the bitmask_tests.cpp source file. The code is trying to set the last remaining bits of a bitmask word by accessing the byte through a device pointer in host code. The end result was not actually being used so it appears the release compile optimized out the entire function. The expected segfault does occur in a debug build.
This PR corrects the logic to copy the last word, set the appropriate bits, and then copy the word back to the device buffer.

[codecov]

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[harrism]

@gpucibot merge