test/framework.py: change logger to return 'e' format.
test/framework.py: change logger to return 'e' format.

* When the framework logs an unexpected sleep time,
  display the units in the same format. Typically,
  it has been returning a float and an 'e'.
  ex. unexpected time.sleep() result - slept for 0.000107049942017s instead of ~4.88758087158e-05s!

* Defer logger interpolation.

Change-Id: I543ad6d41c7f263d61615341437f80973c0017de
Signed-off-by: Paul Vinciguerra <b92f79aabe4c9c18085c7347110a52af0898a0ef@vinciconsulting.com>

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