Starting with OTP-22, Erlang started changing how volume names are handled in
windows: filename:join/1 drops drive letters, filename:split/2 and
file:absname/1 use c:/ as a form instead of c:\\

We adjust the file_utils and their tests accordingly.

Rebar3.14 has new tests that were not windows ready and those are fixed.

Relx has two failing tests that are not covered in this branch.

Tested in Windows 10 with Powershell.

I had misread the code in issue #2200 -- the per-app hooks are still
run, just not the umbrella-level hooks.

Dollar Variable expansion (`$VAR`) was inadvertently disabled for
windows variables, although %VARIABLES% already worked. This reduced the
portability of hooks in general.

Additionally, tests would fail on windows due to bad quoting of paths:
the path C:/a/b/c would fail when passed to the command
`cmd /q /c C:/a/b/c` because it would interpret /a /b and /c as 3
options. Using quotes makes the tests pass.

When REBAR_CONFIG was set it would not effect the top level app's
configuration because app_discover was rereading the top level
rebar.config which ignored REBAR_CONFIG. Instead this patch has
it use the existing configuration from REBAR_CONFIG.

Test required `touch` utility to present on user's machine. Remove this dependency.

Test required `touch` utility to present on user's machine. Remove this dependency.