fix bats warnings

Make sur that PYENV_TEST_DIR is created if mktemp failed
mock python3 usage by updating create_executable function
fix test by adding the right PATH
update path in plugin test
replace for loop with while read
don't use symlink for stub

Requested in https://github.com/pyenv/pyenv/issues/2680
for deployments with a stock `.pyenv-version` that can use any
of a number of Python versions
and for compatibility with `uv`.

* Support `pyenv local --force`
* Support `pyenv-version-file-write --force`
* Support `pyenv version-name --force`
* Ignore missing versions when searching for executables
* Display "commmand not found" even when there are nonexistent versions

* exec.bats: replace `python` and `rspec` with something that doesn't exist globally, either
in Ubuntu Github CI, `python` exists globally

PEP 394 now doesn't require `python` presence or specify what it is.
Tests that invoke it use Py3-specific code.

Using `exec -a` caused Python to use $PATH to look up the full program name (for
`sys.executable`), which 314937d then tried to fix by changing $PATH
also for the system version.
This is not necessary anymore when not using the short name with `exec`.

This was rejected upstream
(https://github.com/rbenv/rbenv/pull/1089#issuecomment-394531896), since
it is not a problem with Ruby apparently.

Uses $PYENV_ROOT to check if system version is used.

Fixes https://github.com/pyenv/pyenv/issues/98.
Fixes https://github.com/pyenv/pyenv/issues/789.

This was imported semi-automatically in 0965577, but `ruby -S` is
different from `python -S`.

`ruby -S`: look for the script using PATH environment variable
`python -S`: don't imply 'import site' on initialization

Use extracted create_hook helper in tests for:

- exec
- hooks
- rehash
- version-name
- version-origin
- which

Spaces in hook path is tested in test/hooks.bats

This new test was creating an (intentionally invalid) .ruby-version file
in current working directory; typically the rbenv project dir.
Immediately after test runs, I had a leftover .ruby-version file.

The version-file tests create and cd into the RBENV_TEST_DIR as part of
setup(). I'm using the same directory for this test fix, but am only
using it for this particular test. None of the other exec tests seem to
need to be in a temp test dir, so no use putting it in setup().

ensure subcommand exists, then include --help in its completion output

This is useful as an indicator where it is coming from.

It was supposed to fix shelling out to Ruby but it in fact broke another
kind of shelling out to Ruby: invoking the `ruby` binary directly with
the `-S` flag.

Fixes #480

This reverts commit db143bb654.

Enables shelling out from a ruby process started with rbenv to a ruby
process with a different RBENV_VERSION. Fixes #121

This removes the workaround created for #15 and solves `ruby -S` support
by setting RUBYPATH. PATH is never changed.

To illustrate how RUBYPATH changes in various configurations:

    PATH=~/bin:~/.rbenv/shims:/usr/bin:/bin
    RBENV_VERSION=1.8 ruby -S rake
    #=> executes ~/.rbenv/versions/1.8/bin/rake
    #=> RUBYPATH=~/bin:~/.rbenv/versions/1.8/bin:/usr/bin:/bin

    RBENV_VERSION=2.0 ruby -S rake
    #=> executes ~/.rbenv/versions/2.0/bin/rake
    #=> RUBYPATH=~/bin:~/.rbenv/versions/2.0/bin:/usr/bin:/bin

    RBENV_VERSION=system ruby -S rake
    #=> executes /usr/bin/rake
    #=> RUBYPATH=~/bin:/rbenv_shims_were_here:/usr/bin:/bin

    RBENV_VERSION=1.8 ruby -S rake
    #=> executes ~/.rbenv/versions/1.8/bin/rake
    #=> RUBYPATH=~/bin:~/.rbenv/versions/1.8/bin:/usr/bin:/bin

closes #379

It was a dumb idea and it wasn't even implemented perfectly.

Per https://github.com/ruby/ruby/blob/7d3db3c/ruby.c#L1383-1391

Fixes #344, #196