[fix tests] link against pthread

[moriarty]

Sorry @mikeferguson this should fix why the CI timed out after merging #752.

adds -lpthread in front of all calls to catkin_add_gtest

With this the tests all pass locally, without they can't find symbols.

I've also put this commit onto my branch for #755

[moriarty]

I see that the restart of the timed out tests passed...
Also, I ran the the tests on the robot and they passed.

So I don't know why on my desktop I needed this extra flag... Other than my desktop being slightly ahead of the robot with a newer gcc

[build] Summary: All 15 packages succeeded!                                                                                                                                                                
[build]   Ignored:   1 packages were skipped or are blacklisted.                                                                                                                                           
[build]   Warnings:  1 packages succeeded with warnings.                                                                                                                                                   
[build]   Abandoned: None.                                                                                                                                                                                 
[build]   Failed:    None.                                                                                                                                                                                 
[build] Runtime: 4 minutes and 15.7 seconds total.                                                                                                                                                         
amoriarty at SW44 in ~/melodic-unique
$ uname -a
Linux SW44 4.15.0-28-generic #30-Ubuntu SMP Thu Jul 12 17:35:57 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
amoriarty at SW44 in ~/melodic-unique
$ gcc --version
gcc (Ubuntu 8-20180414-1ubuntu2) 8.0.1 20180414 (experimental) [trunk revision 259383]
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

[mikeferguson]

I don't think we need this -- CI is working on both your PR and the merged version now (And not working on this PR). Going to close -- please reopen if you find out what is actually going on with your setup. Thanks.

[mfinean]

Also encountered this problem and the above fix worked