test: Simplify data handling for transfer tests

Many of our tests are based around performing transfers of sample data
across passt/pasta created links.  The data flow here can be a bit
hard to follow since, e.g. we create a file transfer it to the guest,
then transfer it back to the host across several different tests.
This also means that the test cases aren't independent of each other.

Because we don't have the original file available at both ends in some
cases, we compare them by generating md5sums at each end and comparing
them, which is a bit complicated.

Make a number of changes to simplify this:
  1. Pre-generate the sample data files as a test asset, rather than
     building them on the fly during the tests proper
  2. Include the sample data files in the mbuto guest image
  3. Because we have good copies of the original data available in all
     contexts, we can now simply use 'cmp' to check if the transfer
     has worked, avoiding md5sum complications.
  4. Similarly we can always use the original copy of the sample data
     on the send side of each transfer, meaning that the tests become
     more independent of each other.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

test/two_guests/basic

@@ -13,7 +13,7 @@
 
 g1tools	ip jq dhclient socat cat
 g2tools	ip jq dhclient socat cat
-htools	ip jq md5sum cut
+htools	ip jq
 
 test	Interface names
 g1out	IFNAME1 ip -j link show | jq -rM '.[] | select(.link_type == "ether").ifname'