support/tests: allow properly indented config fragment

support/tests: allow properly indented config fragment

Currently, defining a config fragment in the runtime test infra requires
that the fragment not to be indented. This is beark, and causes grievance
when looking at the code (e.g. to fix it).

Just strip out all leading spaces/tabs when writing the configuration
lines into the config file, allowing in-line indented config fragments,
like so:

    class TestFoo(bla):
        config = bla.config + \
            """
            FOO=y
            # BAR is not set
            """

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>