cmake, test: Add secp256k1_ prefix to test names

This change improves regex matching options when using `ctest` in
downstream projects.

examples/CMakeLists.txt

@@ -9,7 +9,7 @@ function(add_example name)
     $<$<PLATFORM_ID:Windows>:bcrypt>
   )
   set(test_name ${name}_example)
-  add_test(NAME ${test_name} COMMAND ${target_name})
+  add_test(NAME secp256k1_${test_name} COMMAND ${target_name})
 endfunction()
 
 add_example(ecdsa)