tests: Fix tests for 222c8a43... (mp-modsqrt change).

Ooops.  I accidentally broke some tests (in elliptic curve point-
finding) when I made that change, and (naughty me!) didn't run the full
suite.  I've checked that the breakages are acceptable (i.e., we just
get the positive y-coordinate rather than a random one now) and fixed
the tests.