testing/infra/basetest: move jlevel logic to constructor

testing/infra/basetest: move jlevel logic to constructor

As suggested by Arnout in [1].

While at it, simplify the logic by always appending the BR2_JLEVEL and
defaulting to 0 (the value copied from Config.in is used for 5 years now
and is very unlikely to change).

[1] http://patchwork.ozlabs.org/patch/790525/

Suggested-by: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>