tests/ftest: use string filter() to concatenation cause

tests/ftest: use string filter() to concatenation cause

if, then, else construction is used to concatenate the 'cause' string,
that appends the reason for the test failures.  The reason to use the
checks to TypeError that occurs while concatenating None and str type.
Replace them with a one-liner string.filter(), across the test sources.

Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>