Adding Sphinx sandbox for testing.
Fix for tests returning no output to render at all.
Restructuring the btest directive.

We now process it in two passes: one to save the test at parse time,
and one later to execute once everything has been parsed. No
functionality change, this will later enable defining tests in
multiple parts.
Adding support for splitting tests into multiple parts.
Merge remote-tracking branch 'origin/master' into topic/robin/parts

Includes tests for parts.

Conflicts:
	btest
Documenting parts.

    One can split a single test across multiple files by adding a
    numerical ``#<n>`` postfix to their names, where each ``<n>``
    represents a separate part of the test. ``btests`` will combine
    all of a test's parts in numerical order and execute them
    subsequently within the same sandbox.

Example in the README.
When running a command, TEST_PART now contains the current part number.
Support for parts.

If multiple btest directives reference the same test name, each will
turn into a part of a single test.
Adding tests for Sphinx functionality.
README update.
Fix for parts without any output, plus fixing some tests.
Merge remote-tracking branch 'origin/topic/robin/parts'

* origin/topic/robin/parts:
  Fix for parts without any output, plus fixing some tests.
  README update.
  Adding tests for Sphinx functionality.
  Support for parts.
  When running a command, TEST_PART now contains the current part number.
  Documenting parts.
  Adding support for splitting tests into multiple parts.
  Restructuring the btest directive.
  Fix for tests returning no output to render at all.
  Adding Sphinx sandbox for testing.
Fixing tests.
Merge branch 'topic/robin/parts'