Fix tests being skipped due to executables not found after ECM min bump

ECM, when requiring ECM >=5.38, sets CMAKE_*_OUTPUT_DIRECTORY, so the
tests executables are no longer generated in the current binary dir.

Calling add_test() with the signature
    add_test(<name> <command> [...])
will not result in any further processing of the <command> argument,
it will be executed as is. Using instead
    add_test(NAME <name> COMMAND <command> [...])
will result in <command> getting some handling, cmp. CMake docs:
"If <command> specifies an executable target (created by add_executable())
it will automatically be replaced by the location of the executable
created at build time."
Which is what is needed now here (and also used in ecm_add_test).