Add possibility to run tests directly from MacPorts

This makes it possible to use the command

port test plumed

to compile and test plumed.

Notice that tests on travis are still performed explicitly,
I plan to fix this in the near future, after having verified that there
are no problems with timings.

macports/Portfile.in

@@ -92,3 +92,8 @@ pre-configure {
   configure.ldflags-append ${linalglib}
 }
 
+# Allow running tests from MacPorts
+test.run yes
+test.target check
+
+