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.
This fixes the issue that "idf.py partition-table" had to be run
manually in order for the partition table to be generated, when
building for linux target.
