[NFC] Execution Tests: Long Vector tests update template parameter names (microsoft#7735)

This PR shortens template argument names to shorter standard names.
'DataTypeT' -> 'T'.

In earlier revisions of these tests there were multiple template
arguments being used. The template argument names were made more
descriptive but since then the code has been simplified. The longer
names have been driving me a little nuts. So this PR shortens them back
to standard template argument names again.

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Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

Author: alsepkow