For policy-specific tests, use the version before the policy was
introduced.  Otherwise, use 3.5 where possible.

Also, remove `cmake_minimum_required()` and `project()` calls from
individual cases where they are handled by `CMakeLists.txt`.

Use 3.3 or 2.8.12 where possible.

Make sure that we also trigger variable watches when a variable
is set in the parent scope.

Fixes: #17827

This tests adding a variable_watch inside the callback to an existing
callback

Set the minimum required version of CMake high enough to avoid the
warning for CMAKE_LEGACY_CYGWIN_WIN32.  The warning appears on stderr
and breaks the expected output matching.

Use makefile->IssueMessage() to print the unprocessed watch message in a
format consistent with other CMake messages and with a more complete
call stack for the access.

When a watch does not specify a command to call then variable_watch
prints out a message to stderr.  Remove code after that which collects
all variable values to construct a message that is never printed.
Otherwise such code causes a READ_ACCESS watch to trigger on all
variables in the currents scope.

Reported-by: Yichao Yu <yyc1992@gmail.com>

Add a RunCMake.variable_watch watch test to cover the basic use case.