VPP-1508: Have test/Makefile honor PYTHON= envvar.
VPP-1508: Have test/Makefile honor PYTHON= envvar.

Change-Id: Iff71a5982b73c97e3f546beb1cf52935a267f5f5
Signed-off-by: Paul Vinciguerra <b92f79aabe4c9c18085c7347110a52af0898a0ef@vinciconsulting.com>

vpp-dev/vpp,vpp-dev/vpp,vpp-dev/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,vpp-dev/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,vpp-dev/vpp,FDio/vpp,vpp-dev/vpp,FDio/vpp,FDio/vpp,vpp-dev/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,FDio/vpp