[NF] Tests for OSS_Filter_Filesize

Nerijus - can you finish these off for B and GB please.
[IM] Various improvements - see below

 * used floats for multipliers
 * added setter / getter functions for the multiplier and moving validation there also
 * removed dash (-) from exclusion string... can't see any reason for this
 * removed floor() from return
 * update unfilter() to trim tailing zeros from floats
[IM] Better variable naming