init.sh: automatically accommodate programs with the .exe suffix

init.sh: automatically accommodate programs with the .exe suffix

Automatically arrange for an invocation of "prog" to execute the
program named "prog$EXEEXT" (usually prog.exe).  Thus, all invocations
may use the simpler "prog", yet still work when built on a system
that requires specifying the added suffix.
Do this by constructing a function named "prog" that invokes
"prog.exe" for each .exe file in selected directories.
* tests/init.sh (find_exe_basenames_): New function.
(create_exe_shim_functions_): New function.
(path_prepend_): Use it.