SYNTH: Renamed "filter" to "Vcf"

This is an analogy with generator/Vco, and easier than the previous filter
as a filter wrapper.
All relevant renaming changes done too.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
TESTS: Prefixed all tests files with "tests_"

This reverts (partially, hence the new commit) commit
4418b7c109233d10920aa5c4472cf103471e3612

It has been proved easier to use this way.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>