test_realm_export: Add coverage for tarball upload and retrieval.
test_realm_export: Add coverage for tarball upload and retrieval.

These updates are added as a direct result of the new strategy related
to the the following refactorings:

* Having `do_export_realm` return the value of the tarball path.
See 6e187e974a4e6282d3616312bdfa19d0d2a949d1.

* Moving the upload logic for s3 and local tarball storage out of
`export_realm_wrapper` and into `upload.py`.
See f1041e1fb6cb60f2c53b294695245e4c86a4d40b.

tommyip/zulip,zulip/zulip,tommyip/zulip,kou/zulip,eeshangarg/zulip,rht/zulip,showell/zulip,showell/zulip,kou/zulip,punchagan/zulip,eeshangarg/zulip,synicalsyntax/zulip,brainwane/zulip,rht/zulip,eeshangarg/zulip,hackerkid/zulip,hackerkid/zulip,rishig/zulip,zulip/zulip,kou/zulip,hackerkid/zulip,showell/zulip,punchagan/zulip,shubhamdhama/zulip,hackerkid/zulip,rishig/zulip,zulip/zulip,kou/zulip,shubhamdhama/zulip,timabbott/zulip,eeshangarg/zulip,timabbott/zulip,brainwane/zulip,andersk/zulip,rht/zulip,zulip/zulip,shubhamdhama/zulip,shubhamdhama/zulip,synicalsyntax/zulip,punchagan/zulip,rht/zulip,zulip/zulip,zulip/zulip,hackerkid/zulip,tommyip/zulip,tommyip/zulip,rht/zulip,rht/zulip,rishig/zulip,andersk/zulip,tommyip/zulip,brainwane/zulip,synicalsyntax/zulip,eeshangarg/zulip,kou/zulip,hackerkid/zulip,punchagan/zulip,showell/zulip,synicalsyntax/zulip,hackerkid/zulip,rishig/zulip,synicalsyntax/zulip,timabbott/zulip,timabbott/zulip,showell/zulip,timabbott/zulip,andersk/zulip,tommyip/zulip,zulip/zulip,rishig/zulip,showell/zulip,timabbott/zulip,shubhamdhama/zulip,punchagan/zulip,shubhamdhama/zulip,synicalsyntax/zulip,tommyip/zulip,brainwane/zulip,shubhamdhama/zulip,showell/zulip,eeshangarg/zulip,timabbott/zulip,andersk/zulip,rht/zulip,rishig/zulip,andersk/zulip,brainwane/zulip,andersk/zulip,rishig/zulip,brainwane/zulip,synicalsyntax/zulip,kou/zulip,kou/zulip,andersk/zulip,punchagan/zulip,brainwane/zulip,eeshangarg/zulip,punchagan/zulip