ENVIRONMENT_MODIFICATION
------------------------

Specify environment variables that should be modified for running a test. Note
that the operations performed by this property are performed after the
:prop_test:`ENVIRONMENT` property is already applied.

If set to a list of environment variables and values of the form
``MYVAR=OP:VALUE``. Entries are considered in the order specified in the
property's value. The ``OP`` may be one of:

  - ``reset``: Reset to the unmodified value, ignoring all modifications to
    ``MYVAR`` prior to this entry. Note that this will reset the variable to
    the value set by :prop_test:`ENVIRONMENT`, if it was set, and otherwise
    to its state from the rest of the CTest execution.
  - ``set``: Replaces the current value of ``MYVAR`` with ``VALUE``.
  - ``unset``: Unsets the current value of ``MYVAR``.
  - ``string_append``: Appends ``VALUE`` to the current value of ``MYVAR``.
  - ``string_prepend``: Prepends ``VALUE`` to the current value of ``MYVAR``.
  - ``path_list_append``: Appends ``VALUE`` to the current value of ``MYVAR``
    using the platform-specific list separator.
  - ``path_list_prepend``: Prepends ``VALUE`` to the current value of
    ``MYVAR`` using the platform-specific list separator.
  - ``cmake_list_append``: Appends ``VALUE`` to the current value of ``MYVAR``
    using ``;`` as the separator.
  - ``cmake_list_prepend``: Prepends ``VALUE`` to the current value of
    ``MYVAR`` using ``;`` as the separator.

Unrecognized ``OP`` values will result in the test failing before it is
executed. This is so that future operations may be added without changing
valid behavior of existing tests.

The environment changes from this property do not affect other tests.
