This is part 2 of a larger PR, ocaml#12085, which includes a part 1
(compiler and debugger changes), this part, and a third part which is
tests of part 1 that make use of (and hence motivate and test) these
changes. Please see that PR for example uses of these changes.

1. Add the ability to have scripts or files with ocamltest variables
references expanded. Added a new "expand" action to ocamltest. It is
mostly like the "copy" action, but the source file is read a line at a
time and the lines are expanded. It does not support source
directories, but the destination can be a directory.

2. Enhanced ocamltest to have a facility for making builtin
functions. These are like ocamltest variables but have a function
attached to them, and when the variable is expanded, the arguments are
first expanded and then the function is called and its result is
returned. Currently they only take one parameter, but it should not be
too hard to add the ability to have multiply arguments.

3. Added a new "dumpenv_expanded" action which not
only shows the value of each variable, but also what
they expand to. When variables are originally
assigned, their RHS is not expanded, but only
later when the variable itself is looked up.

4. Two builtin functions were defined:

- bppm_decode does BUILD_PATH_PREFIX_MAP decoding

- bppm_encode does BUILD_PATH_PREFIX_MAP encoding

See https://reproducible-builds.org/specs/build-path-prefix-map/

5. Prepare to be able to do Dune tests:

5a. Added a action helper for testing if a program
is available in PATH.

5b. Use the helper to make an action, has_dune,
that detects whether dune is available.