add mactime plugin unit / code coverage tests

add mactime plugin unit / code coverage tests

The unit and code coverage tests are boring. The rest of the patch
involves test and packet-generator infra cleanups.

Teach the "make test-xxx" family of targets to set the api test plugin
path correctly, to make "binary-api <api-message-name> <args>" debug
CLI commands work correctly in the "make test"
environment. Unfortunately involves both the top-level and test
Makefiles.

Add a minor pg cli feature, a CLI to manually set
s->sw_if_index[VLIB_TX].

Consider the case where one configures an interface with both a
device-input and an output feature. To test the output feature using
the pg, it's necessary to inject packets into the interface output
node with both b->sw_if_index[VLIB_TX] and b->sw_if_index[VLIB_RX] set
correctly. For example:

packet-generator new {
 name tx
 limit 15
 size 128-128
 interface local0  # rx: device input feature not configured on local0
 tx-interface loop0 # tx: output node requires b->sw_if_index[VLIB_TX]
 node loop0-output
 data {
   hex 0x01005e7ffffa000dead0000008000102030405060708090a0b0c0d0e0f0102030405
   }
}

Fix a longstanding bug in the packet generator stream setup.  Remove
kludges which set b->sw_if_index[VLIB_TX] to ~0 [in multiple places]
instead of using the stream value s->sw_if_index[VLIB_TX], and setting
THAT datum correctly.

Change-Id: I1097a18e8db73661ded6b822c1d718f7e5cf36ed
Signed-off-by: Dave Barach <dave@barachs.net>