[tests] Much simpler approach to ensure test_machdep.i is always in test dir

tests/fc_script/test_machdep.i

 /* run.config
    NOFRAMAC: Just test the generation of a custom machdep with the installed script.
    COMMENT: No C code gets analyzed there. File is empty on purpose
-   COMMENT: WARNING: be sure to keep the ENABLED_IF below in sync with test_machdep_dummy.i
-   COMMENT: a disabled file is not copied, and some _other_ tests contain in their oracles
-   COMMENT: the number of .{ci} files in the directory...
+   COMMENT: be sure to keep the first EXECNOW: it ensures
+   COMMENT: that dune will copy the file regardless of the environment
+   COMMENT: so that other tests whose oracles depend on the number of file in
+   COMMENT: the directory will be stable.
+   EXECNOW: LOG empty.res touch empty.res
    ENABLED_IF: (and %{bin-available:clang} %{bin-available:yq})
    FILTER: sed -e '/^version:/d'
    EXECNOW: LOG custom_machdep.yaml LOG make_machdep.err.log PTESTS_TESTING=1 %{bin:frama-c-script} make-machdep --compiler clang --cpp-arch-flags='--target=x86_64' | yq -Y 'del(.version)|del(.custom_defs)' > custom_machdep.yaml 2> make_machdep.err.log

tests/fc_script/test_machdep_dummy.i

-/* run.config
-   NOFRAMAC: Just test the generation of a custom machdep with the installed script.
-   COMMENT: This is just a placeholder file for test_machdep.i when clang or yq is unavailable.
-   COMMENT: In that case, we don't do the test, but still need to copy a .i file
-   COMMENT: to keep the number of files inspected by fc-script identical.
-   COMMENT: WARNING: be sure to keep the ENABLED_IF below in sync with test_machdep.i
-   ENABLED_IF: (or (not %{bin-available:clang}) (not %{bin-available:yq}))
-   EXECNOW: LOG test.res touch test.res
-*/