Un-xfail/move/delete some tests.
Tests for fixed issues.

Closes #3794.
Closes #4025.
Closes #5688.
Closes #5708.
Closes #7012.
Closes #7327.
Re-xfail extern-pass-TwoU{8,16}.rs (still doesn't work on 32-bit platforms).
auto merge of #8932 : huonw/rust/closed-issues, r=thestinger