test.py: run Alternator test with the correct Scylla binary
test.py: run Alternator test with the correct Scylla binary

The Alternator test's run script, test/alternator/run, runs Scylla.
By default, it chooses the last built Scylla executable build/*/scylla.

However, test.py has a "mode" option, that should be able to choose which
build mode to run. Before this patch, this mode option wasn't honored by
the Alternator test, so a "test.py alternator/run" would run the same
Scylla binary (the one last built) three times, instead of running each
of the three build modes.

We fix this in this patch: test.py now passes the "SCYLLA" environment
variable to the test/alternator/run script, indicating the location of the
Scylla binary with the appropriate build mode. The script already supported
this environment variable to override its default choice of Scylla binary.

In test.py, we add to the run_test() function an optional "env" parameter
which can be used to pass additional environment variables to the test.

Fixes #6286

Signed-off-by: Nadav Har'El <625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>
Message-Id: <20200427131958.28248-1-625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>

scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla