Make the tests run faster by reducing PREC

[certik]

This speeds up the test_series_generic test in Debug mode from 1.09s to 0.83s.
Before:

$ ctest -R test_series_generic
Test project /home/certik/repos/symengine
    Start 6: test_series_generic
1/1 Test #6: test_series_generic ..............   Passed    1.09 sec

100% tests passed, 0 tests failed out of 1

Total Test time (real) =   1.10 sec

After:

$ ctest -R test_series_generic
Test project /home/certik/repos/symengine
    Start 6: test_series_generic
1/1 Test #6: test_series_generic ..............   Passed    0.83 sec

100% tests passed, 0 tests failed out of 1

Total Test time (real) =   0.84 sec

This helps with #1012, but still not fixed.

[certik]

I rebased on top of the latest master. The tests should pass now.

[isuruf]

I don't think the test time is a big issue. Were large prec used to test some specific code paths?

[certik]

Were large prec used to test some specific code paths?

That's what I am not sure --- it doesn't seem so. And if it doesn't test anything useful, I would argue the test should be as fast as possible. As we will keep adding more tests in the future, every bit counts.