Cleanup and rearrange tests
Sync & cleanup tst/check.g and tst/testall.g

These two really should be merged back together
Pass ring directly to _SI_CallFunc1/2/3/M without using SI_SetCurrRing

This partially addresses issues #39 and #40: It gets rid of a primary
source of SI_SetCurrRing uses, and at the same time tightens down
which "active rings" are seen by these calls: Now any stray "currRing"
value is cleared, and thus, a command like
  SI_nc_algebra(-1,0)
will fail with an error. You have to instead write e.g.
  SI_nc_algebra(SI_number(r,-1),0);