doc: ztest: various fixes

This page is:

- using literal blocks [1] where it should be using code
  blocks [2], which won't produce the desired results [3]

- missing required whitespace between code block directives and their
  pygments syntax arguments [4]

  In other words, this:

    .. code-block::foo

  should be:

    .. code-block:: foo

- using an incorrect syntax (option:`CONFIG_FOO` or
  :kconfig:option:`CONFIG_FOO=y1) to refer to Kconfig
  options (should be :kconfig:option:`CONFIG_FOO`, etc)

Fix these issues so we get syntax-highlighted code blocks in the
correct pygments highlight syntax and valid references to Kconfig
options.

Adjust directive content indentation to match the usual .rst style and
make some grammar tweaks as well.

[1] https://www.sphinx-doc.org/en/master/usage/restructuredtext/basics.html#literal-blocks

[2] https://www.sphinx-doc.org/en/master/usage/restructuredtext/directives.html#directive-code-block

[3] Use of literal blocks can produce the same results if there is a
    consistent choice of highlight language specified for the file, but
    this file is intermixing code blocks that should be highlighted as
    either C or shell commands, so there is no single choice of
    highlight syntax that works.

[4] https://docutils.sourceforge.io/docs/ref/rst/restructuredtext.html#directives



Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>