tests: Fix 'test-wipe'

tests: Fix 'test-wipe'

Type: fix

Error:
vagrant@ ~/vpp (make-test-wipe) $ make test-wipe
make[1]: Entering directory '/home/vagrant/vpp/test'
make[2]: Entering directory '/home/vagrant/vpp/test'
make[2]: *** ext: No such file or directory.  Stop.
make[2]: Leaving directory '/home/vagrant/vpp/test'
make[1]: *** [Makefile:239: wipe] Error 2
make[1]: Leaving directory '/home/vagrant/vpp/test'
make: *** [Makefile:440: test-wipe] Error 2

Signed-off-by: Neale Ranns <neale@graphiant.com>
Change-Id: I3615a962882d5d56c394aa628e9942fed07cde84