Fix strange testing issues

[mizzao]

When running tests, sometimes we see pages of the following, which causes the tests to hang:

I20170512-16:55:03.489(-4)? Tried to set group for nonexistent user puwvy24Qea7oHeucR
I20170512-16:55:03.491(-4)? Tried to set group for nonexistent user xjPCt6uZZLALBTz66
I20170512-16:55:03.492(-4)? Tried to set group for nonexistent user TLqxntWGJADLEQ9Cn
I20170512-16:55:03.494(-4)? Tried to set group for nonexistent user dCDgHxB2q6QsafNci
I20170512-16:55:03.496(-4)? Tried to set group for nonexistent user kGrPP6KCDLTjHeKcq
I20170512-16:55:03.498(-4)? Tried to set group for nonexistent user rJbbywYZz2dtskdxF
I20170512-16:55:03.499(-4)? Tried to set group for nonexistent user x8YTQx4TjnrbbWCEp
I20170512-16:55:03.501(-4)? Tried to set group for nonexistent user DDfsj7QxWpPazZuCL
I20170512-16:55:03.503(-4)? Tried to set group for nonexistent user gzk86Kj6HEd4k9QTg
I20170512-16:55:03.504(-4)? Tried to set group for nonexistent user H4wD76avj67GCyCqj
I20170512-16:55:03.506(-4)? Tried to set group for nonexistent user wntksPPDWgZ9Goqn9
I20170512-16:55:03.508(-4)? Tried to set group for nonexistent user gDngZZs6FfjXtDPwp
I20170512-16:55:03.510(-4)? Tried to set group for nonexistent user 6FJ8aiDjmkgN8WNFo
I20170512-16:55:03.511(-4)? Tried to set group for nonexistent user 2BcRJDETGbzgBNYzD

This is especially prone to breakage on Linux x64 phantomjs, hence the hack of .travis.yml to run on OS X. However, this makes the tests run slow and is brittle.

This pops up randomly when restarting tests in meteor test-packages .... It also is replicable by running the phantomjs command that Travis starts, manually.