Only build and run tests in variant=coverage

example/echo-op/Jamfile

@@ -5,6 +5,8 @@
 # file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 #
 
-exe echo-op/echo-op :
+exe echo-op :
     echo_op.cpp
+    :
+    <variant>coverage:<build>no
     ;