crypto/internal/fips140: remove key import PCTs, make keygen PCTs fatal

crypto/internal/fips140: remove key import PCTs, make keygen PCTs fatal

CMVP clarified with the September 2nd changes to IG 10.3.A that PCTs
don't need to run on imported keys.

However, PCT failure must enter the error state (which for us is fatal).

Thankfully, now that PCTs only run on key generation, we can be assured
they will never fail.

This change should only affect FIPS 140-3 mode.

While at it, make the CAST/PCT testing more robust, checking
TestConditional is terminated by a fatal error (and not by t.Fatal).

Fixes #74947
Updates #69536

Change-Id: I6a6a696439e1560c10f3cce2cb208fd40c5bc641
Reviewed-on: https://go-review.googlesource.com/c/go/+/701517
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
TryBot-Bypass: Filippo Valsorda <filippo@golang.org>