Increase user clock measurement delay

[michael-adler]

• Increase delay before measuring user clock after changing frequency. The previous delay was a bit short, leading to measurement errors.
• Correct naming of DELAY time macro units from ms to us.

[coveralls]

Pull Request Test Coverage Report for Build 6000138082

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 15 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.06%) to 73.652%

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Files with Coverage Reduction	New Missed Lines	%
libraries/plugins/xfpga/open.c	1	74.68%
libraries/plugins/uio/opae_uio.c	4	90.18%
libraries/plugins/vfio/opae_vfio.c	4	85.13%
libraries/plugins/xfpga/mmio.c	6	82.87%
<!--	Total:	15		-->

Totals
Change from base Build 5978741746:	-0.06%
Covered Lines:	15741
Relevant Lines:	21372

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💛 - Coveralls