FCT tendency tests swallow failures; FCTHNewInv is inf #573

The FCT checks in TendencyTermsTest (added in #447) can never fail CTest, and several of them are failing silently.

• testFCTTracerHorzAdvOnCell sets Err = 0; after each check, so it always returns 0 and failures only appear as [error] lines in the test log.
• FCTHNewInv is inf on the plane, single-precision plane and sphere meshes.
• In TEND_PLANE_SINGLE_PRECISION_TEST, FCTHProv, FCTHProvInv, FCTHighAndLowOrderFlux_High and FCTAccumulateHighOrderFlux_0 also fail (e.g. FCTHProv expected 0, got 1.5e-05).

Seen on chrysalis/intel with develop at 480467b; grep FCT test/logs/TEND_*.log in a build tree shows them.

@overfelt, could you take a look?

Posted by Claude Code on @xylar's behalf. The testing, analysis and wording above are AI-authored; please check them accordingly.