enable test magit-blame-mode

I cannot reproduce the behaviour Yann saw that lead to it being
disabled.
tests/*.el: add headings
comment out tests that require mocker

When emacs is started in batch mode magit-init-test-expansion causes
strange errors.  In non-batch mode there is no problem.

  emacs -batch -Q -l tests/run-test.el
  Invalid read syntax: "#"

  emacs -batch -Q -f toggle-debug-on-error -l tests/run-test.el
  Symbol's value as variable is void: <

Trying to compile the tests results in errors about mocker symbols not
being defined.
when running tests: ensure previous git process is done

When the new variable `magit-process-force-synchronous' is non-nil
`magit-run*' runs all processes synchronously.  This is necessary
because when running tests on travis it can happen that the previous
git process hasn't finished yet when an attempt is made to creating
the next one.  This causes tests to fail randomly on travis, depending
on general load.

This new variable should only be used for its intended purpose.
magit-tests-section-has-item-title: fail more gracefully

That is fail inside should instead of raising an error.
The error is caused (triggered) by the last commit,
why I don't know.