tests: don't test spans for equality in FormFactorBasicTest

Causes failures on various architectures, https://buildd.debian.org/status/fetch.php?pkg=bornagain&arch=arm64&ver=20.2%2Bds3-1&stamp=1690724032&raw=0

assigned to @j.wuttke

changed milestone to %v21

added numerics label

added bug label

mentioned in commit 9618bd28

mentioned in merge request !1817 (merged)

mentioned in commit a524522d

mentioned in commit 5ae1f688

closed with merge request !1817 (merged)