Avoid unnecessary rebuilds of test targets

Currently, all targets created by `pytest_discover_tests` will cause
a rebuild *every time* `make` / `ninja` is called, even if no files
have changed since the last build.

Quoting the CMake documentation for `add_custom_target`:

> The target has no output file and is always considered out of
> date even if the commands try to create a file with the name of
> the target. Use the add_custom_command() command to generate a
> file with dependencies.

This change should be functionally equivalent, but avoids the
indefinite rebuilds because the target will only check for the
presence of `${_tests_file}` rather than unconditionally building
it.

Author: mhx

cmake/FindPytest.cmake

@@ -105,10 +105,9 @@ if (Pytest_FOUND AND NOT TARGET Pytest::Pytest)
         set(_include_file "${CMAKE_CURRENT_BINARY_DIR}/${NAME}_include.cmake")
         set(_tests_file "${CMAKE_CURRENT_BINARY_DIR}/${NAME}_tests.cmake")
 
-        # Create a custom target to run the tests.
-        add_custom_target(
-            ${NAME} ALL VERBATIM
-            BYPRODUCTS "${_tests_file}"
+        add_custom_command(
+            VERBATIM
+            OUTPUT "${_tests_file}"
             DEPENDS ${_DEPENDS}
             COMMAND ${CMAKE_COMMAND}
             -D "PYTEST_EXECUTABLE=${PYTEST_EXECUTABLE}"
@@ -127,6 +126,9 @@ if (Pytest_FOUND AND NOT TARGET Pytest::Pytest)
             -D "CTEST_FILE=${_tests_file}"
             -P "${CMAKE_CURRENT_FUNCTION_LIST_DIR}/PytestAddTests.cmake")
 
+        # Create a custom target to run the tests.
+        add_custom_target(${NAME} ALL DEPENDS ${_tests_file})
+
           file(WRITE "${_include_file}"
               "if(EXISTS \"${_tests_file}\")\n"
               "    include(\"${_tests_file}\")\n"