Make test suite work on macos

[le-lenn]

AI disclaimer

This PR was partly created with codex cli.

What changed

• Add an explicit start path to the top-level directory scan: find . $find_args
  so BSD/macOS find doesn’t treat -mindepth/-maxdepth as illegal options when
  no path is provided.
• Replace GNU-only -executable with portable -perm -u+x for runnable test
  discovery.

Why

• BSD/macOS find requires a path argument; without it, options like -mindepth
  are parsed as invalid (BSD find(1) synopsis and option descriptions). GNU
  find allows no path, so this only fails on BSD/macOS.
• -executable is documented in GNU findutils but is not listed in BSD find(1).
  -perm -u+x is supported by both and checks the user execute bit.

Flag details (with docs)

• . start path
  • BSD find(1) synopsis requires a path before expressions: https://
    man.freebsd.org/cgi/man.cgi?query=find&sektion=1
• -mindepth 1 / -maxdepth 1
  • GNU: https://www.gnu.org/software/findutils/manual/html_node/find_html/
    Directories.html
  • BSD: https://man.freebsd.org/cgi/man.cgi?query=find&sektion=1
• -type d / -type f
  • GNU: https://www.gnu.org/software/findutils/manual/html_node/find_html/
    Type.html
  • BSD: https://man.freebsd.org/cgi/man.cgi?query=find&sektion=1
• -name
  • GNU: https://www.gnu.org/software/findutils/manual/html_node/find_html/
    Base-Name-Patterns.html
  • BSD: https://man.freebsd.org/cgi/man.cgi?query=find&sektion=1
• -perm -u+x
  • GNU -perm semantics: https://www.gnu.org/software/findutils/manual/
    html_node/find_html/Mode-Bits.html
  • BSD -perm semantics: https://man.freebsd.org/cgi/man.cgi?
    query=find&sektion=1
• GNU-only -executable (reason for change)
  • GNU doc: https://www.gnu.org/software/findutils/manual/html_node/
    find_html/Mode-Bits.html
  • BSD find(1) does not list -executable: https://man.freebsd.org/cgi/
    man.cgi?query=find&sektion=1

Behavior

• Directory discovery still targets only direct child directories.
• Script discovery still targets executable test scripts; the criteria now work
  on both GNU and BSD/macOS..

[m90]

This is brilliant, thanks! I assume you tested this on MacOS, it works as before for me in any case.