Fix and UnXFAIL tests in test_expbug4_failing(); see PR #757
Fix and UnXFAIL test in test_expbug5_failing(); see PR #757
Drop test_series1_failing() introduced in PR #757, see test_simple1()
Revert "Drop test_series1_failing() introduced in PR #757, see test_simple1()"

This reverts commit 754f3aa44b1e695e8195273232f2ad3e4cfae5da.
Merge pull request #2407 from skirpichev/unxfail-nseries-tests

UnXFAIL nseries tests