Testserver: SUMUP_READ and SUMUP_WRITE with variable length

[stlehmann]

This PR allows to have variable length sumread and sumwrite requests with the testserver. To keep things simple the testserver handles only uint8 and str data. The returned string is always "test".

[coveralls]

Pull Request Test Coverage Report for Build 620

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 4 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.3%) to 94.108%

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Files with Coverage Reduction	New Missed Lines	%
pyads/pyads_ex.py	4	89.55%
<!--	Total:	4		-->

Totals
Change from base Build 609:	-0.3%
Covered Lines:	1102
Relevant Lines:	1171

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 624

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.09%) to 94.364%

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Files with Coverage Reduction	New Missed Lines	%
pyads/pyads_ex.py	1	90.3%
<!--	Total:	1		-->

Totals
Change from base Build 609:	-0.09%
Covered Lines:	1105
Relevant Lines:	1171

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💛 - Coveralls