Update random seed functionality in tests (again)

[segasai]

I've restructured the seed settings in the tests, so now it's centralized in conftest.py also now it's possible to run tests with multiple seed values if needed I.e.

for a in `seq 10 ` ; do 
 env DYNESTY_TEST_RANDOMSEED=$a PYTHONPATH=py:tests:$PYTHONPATH pytest  --full-trace   tests/test_printing.py  ;
 done 

This can be useful to catch rare/sporadic errors. While the main tests will still use one single seed.

[coveralls]

Pull Request Test Coverage Report for Build 856767376

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 71.97%

---

Totals
Change from base Build 855437781:	0.04%
Covered Lines:	3379
Relevant Lines:	4695

---

💛 - Coveralls