Set num_workers=0 in all tests

[HamedHemati]

This PR sets num_workers=0 in test_stratgies.py and test_plugins.py to avoid data loading issues when running tests locally.
It also adds num_workers param to AGEMPlugin.

[coveralls]

Pull Request Test Coverage Report for Build 1813523162

• 6 of 6 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.003%) to 77.869%

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Totals
Change from base Build 1813020939:	0.003%
Covered Lines:	11622
Relevant Lines:	14925

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💛 - Coveralls