ARROW-2318: [Plasma] Run plasma store tests with unique socket

[pcmoritz]

It seems like using the same store socket name for different tests introduced race conditions between connecting clients and starting/stopping the plasma store for each tests. This PR should fix it.

This should fix https://issues.apache.org/jira/browse/ARROW-2318

[wesm]

+1

Seems like these tests are "outrunning" the Plasma store getting ready. No big deal, but these stderr warnings might be a tad aggressive

$ ./release/client_tests 
[==========] Running 7 tests from 1 test case.
[----------] Global test environment set-up.
[----------] 7 tests from TestPlasmaStore
[ RUN      ] TestPlasmaStore.DeleteTest
Connection to IPC socket failed for pathname /tmp/store2961491687, retrying 50 more times
[       OK ] TestPlasmaStore.DeleteTest (103 ms)
[ RUN      ] TestPlasmaStore.ContainsTest
Connection to IPC socket failed for pathname /tmp/store4069682055, retrying 50 more times
[       OK ] TestPlasmaStore.ContainsTest (103 ms)
[ RUN      ] TestPlasmaStore.GetTest
Connection to IPC socket failed for pathname /tmp/store4275793320, retrying 50 more times
[       OK ] TestPlasmaStore.GetTest (103 ms)
[ RUN      ] TestPlasmaStore.MultipleGetTest
Connection to IPC socket failed for pathname /tmp/store1316524434, retrying 50 more times
[       OK ] TestPlasmaStore.MultipleGetTest (104 ms)
[ RUN      ] TestPlasmaStore.AbortTest
Connection to IPC socket failed for pathname /tmp/store764105752, retrying 50 more times
[       OK ] TestPlasmaStore.AbortTest (103 ms)
[ RUN      ] TestPlasmaStore.MultipleClientTest
Connection to IPC socket failed for pathname /tmp/store2758522039, retrying 50 more times
[       OK ] TestPlasmaStore.MultipleClientTest (104 ms)
[ RUN      ] TestPlasmaStore.ManyObjectTest
Connection to IPC socket failed for pathname /tmp/store3072601515, retrying 50 more times
[       OK ] TestPlasmaStore.ManyObjectTest (124 ms)
[----------] 7 tests from TestPlasmaStore (745 ms total)

[----------] Global test environment tear-down
[==========] 7 tests from 1 test case ran. (745 ms total)
[  PASSED  ] 7 tests.