Rename target test -> tests
Rename target test -> tests

So that we do not conflict with possibly pre-defined targets; see CMake
policy CMP0037.

otris/ews-cpp,otris/ews-cpp,bkircher/ews-cpp,bkircher/ews-cpp,m-Schlitzer/ews-cpp,m-Schlitzer/ews-cpp,bkircher/ews-cpp,m-Schlitzer/ews-cpp,otris/ews-cpp,otris/ews-cpp