Use pytest plugin mechanism to conditionally skip tests requiring Julia runtime

[tkf]

This PR adds pytest plugin to provide:

• julia fixture which automate initializing Julia runtime used for tests.

• Tests using julia fixture or other fixtures which in turn depend on julia fixture can be skipped with --no-julia CLI option.

This plugin can be used not only for PyJulia's test suite but also for any downstream projects.

TODO:

• rebase after Refactoring #235

[coveralls]

Pull Request Test Coverage Report for Build 569

• 123 of 149 (82.55%) changed or added relevant lines in 4 files are covered.
• 7 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.1%) to 75.515%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
julia/with_rebuilt.py	1	4	25.0%
julia/pytestplugin.py	4	12	33.33%
julia/core.py	117	132	88.64%

Files with Coverage Reduction	New Missed Lines	%
julia/core.py	7	83.05%

Totals
Change from base Build 546:	0.1%
Covered Lines:	660
Relevant Lines:	874

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 572

• 123 of 149 (82.55%) changed or added relevant lines in 4 files are covered.
• 7 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.1%) to 75.515%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
julia/with_rebuilt.py	1	4	25.0%
julia/pytestplugin.py	4	12	33.33%
julia/core.py	117	132	88.64%

Files with Coverage Reduction	New Missed Lines	%
julia/core.py	7	83.05%

Totals
Change from base Build 546:	0.1%
Covered Lines:	660
Relevant Lines:	874

---

💛 - Coveralls