CMake: Exclude tests from the default build.
CMake: Exclude tests from the default build.

Currently, if you run `make`, you will end up building the
`libprocess-tests` binary because we do not exclude it from the default
build.  This commit adds the `EXCLUDE_FROM_ALL` property to the
`libprocess-tests` binary.

The `EXCLUDE_FROM_ALL` property is also added to the test helper
binary `test-linkee`.

`make check` will still run the test.

chhsia0/mesos,reneploetz/mesos,neilconway/mesos,Aman-Jain-14/customizedMesos,chhsia0/mesos,andschwa/mesos,Gilbert88/mesos,craimbert/mesos,kaysoky/mesos,Gilbert88/mesos,reneploetz/mesos,gsantovena/mesos,jpeach/mesos,kaysoky/mesos,kaysoky/mesos,gsantovena/mesos,reneploetz/mesos,verizonlabs/mesos,jpeach/mesos,craimbert/mesos,craimbert/mesos,yuquanshan/customizedMesos,Gilbert88/mesos,jpeach/mesos,dforsyth/mesos,dforsyth/mesos,yuquanshan/customizedMesos,neilconway/mesos,jpeach/mesos,asamerh4/mesos,kaysoky/mesos,abudnik/mesos,chhsia0/mesos,chhsia0/mesos,zmalik/mesos,craimbert/mesos,zmalik/mesos,andschwa/mesos,jpeach/mesos,zmalik/mesos,abudnik/mesos,gsantovena/mesos,asamerh4/mesos,gsantovena/mesos,yuquanshan/customizedMesos,chhsia0/mesos,neilconway/mesos,craimbert/mesos,reneploetz/mesos,neilconway/mesos,verizonlabs/mesos,abudnik/mesos,zmalik/mesos,verizonlabs/mesos,abudnik/mesos,asamerh4/mesos,kaysoky/mesos,Aman-Jain-14/customizedMesos,dforsyth/mesos,reneploetz/mesos,Aman-Jain-14/customizedMesos,gsantovena/mesos,craimbert/mesos,yuquanshan/customizedMesos,Aman-Jain-14/customizedMesos,zmalik/mesos,dforsyth/mesos,chhsia0/mesos,Gilbert88/mesos,chhsia0/mesos,abudnik/mesos,Aman-Jain-14/customizedMesos,verizonlabs/mesos,gsantovena/mesos,yuquanshan/customizedMesos,craimbert/mesos,jpeach/mesos,yuquanshan/customizedMesos,verizonlabs/mesos,Gilbert88/mesos,abudnik/mesos,verizonlabs/mesos,asamerh4/mesos,jpeach/mesos,dforsyth/mesos,asamerh4/mesos,Gilbert88/mesos,Aman-Jain-14/customizedMesos,andschwa/mesos,andschwa/mesos,verizonlabs/mesos,neilconway/mesos,andschwa/mesos,andschwa/mesos,kaysoky/mesos,yuquanshan/customizedMesos,asamerh4/mesos,zmalik/mesos,zmalik/mesos,Gilbert88/mesos,reneploetz/mesos,abudnik/mesos,neilconway/mesos,andschwa/mesos,dforsyth/mesos,gsantovena/mesos,neilconway/mesos,Aman-Jain-14/customizedMesos,kaysoky/mesos,reneploetz/mesos,dforsyth/mesos,asamerh4/mesos