s390: Reorg and extend BFP "convert to/from fixed/logical" testcases

s390: Reorg and extend BFP "convert to/from fixed/logical" testcases

Testing these insns was spread over 3 testcases: rounding-3, fpconv,
and fpext.

rounding-3 was testing "convert to fixed" with rounding per FPC.
fpext was testing "convert to/from logical" with rounding per M3 field.
fpconv was testing "convert to/from fixed" with rounding per M3 field.

The new testcase bfp-convert replaces these 3.  It removes the
"convert from fixed/logical" testing because that is done in bfp-emit.pl.
It consolidates end extends "convert to fixed/logical" testing from
those files.

Testing "convert to logical" has been commented out until the condition
code weirdness for negative input values has been sorted.

Part of fixing https://bugs.kde.org/show_bug.cgi?id=509572