topotests: fix bgp_vpnv4_noretain

topotests: fix bgp_vpnv4_noretain

Fix the following issues:
- two tests are done in one function. Dispatch the tests in two
functions to help the test debug.
- the first test passes even if a third prefix is not filtered. Match
the exact to avoid false positive.
- the expected values contains variable like version. Do no check
variable values.

Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>