Add nidigital clock generator and frequency counter system tests

[ni-jfitzger]

• This contribution adheres to CONTRIBUTING.md.

• I've updated CHANGELOG.md if applicable.

• I've added tests applicable for this pull request

What does this Pull Request accomplish?

• Adds nidigital system test test_clock_generator_abort
• Adds nidigital system test test_clock_generator_generate_clock
• Adds nidigital system test test_frequency_counter_measure_frequency

List issues fixed by this Pull Request below, if any.

What testing has been done?

Ran the new system tests on a system with nidigital 19.0.1 installed.

[codecov]

Codecov Report

Merging #1412 into master will decrease coverage by 0.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1412      +/-   ##
==========================================
- Coverage   91.90%   91.77%   -0.13%     
==========================================
  Files          20       25       +5     
  Lines        3642     5035    +1393     
==========================================
+ Hits         3347     4621    +1274     
- Misses        295      414     +119     

Flag	Coverage Δ
#codegenunittests	88.24% <ø> (ø)
#nidmmsystemtests	95.48% <ø> (?)
#nifakeunittests	96.36% <ø> (ø)
#nimodinstunittests	95.37% <ø> (ø)
#niscopesystemtests	83.15% <ø> (?)
#nisesystemtests	100.00% <ø> (?)
#niswitchsystemtests	98.11% <ø> (?)
#nitclkunittests	95.45% <ø> (ø)

Impacted Files	Coverage Δ
generated/nise/nise/_library.py	100.00% <0.00%> (ø)
generated/niscope/niscope/waveform_info.py	57.35% <0.00%> (ø)
generated/niscope/niscope/_library.py	86.69% <0.00%> (ø)
generated/niswitch/niswitch/_library.py	98.11% <0.00%> (ø)
generated/nidmm/nidmm/_library.py	95.48% <0.00%> (ø)

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