fix(host_test): use ${project_elf} variable and keyword link form

Three host_test CMakeLists.txt files relied on idioms tied to a
specific build system layout. Make them portable:

- spiffs/host_test and esp_partition/host_test/partition_api_test
  passed a hard-coded "<project>.elf" target name to add_dependencies().
  Use ${project_elf}, the canonical variable that resolves to the live
  executable target name in either build system.

- nvs_flash/host_test/nvs_page_test linked --coverage using the plain
  signature of target_link_libraries. CMake forbids mixing plain and
  keyword signatures on the same target; the component library link
  already uses the keyword form. Switch the --coverage link to the
  keyword signature.

components/spiffs/host_test/CMakeLists.txt

@@ -23,4 +23,4 @@ set_property(
     APPEND PROPERTY ADDITIONAL_CLEAN_FILES "${build_dir}/image.bin")
 
 
-add_dependencies(host_test_spiffs.elf image.bin)
+add_dependencies(${project_elf} image.bin)