support/testing: fix run-tests -j

support/testing: fix run-tests -j

Since commit cf3cd4388a652c9af27ef1c35622e2d0a55b99a9 the -j option is
silently ignored.

The configuration lines are processed using '\n'.join().
This function adds intervening occurrences of the separator, but the
resulting string does not end at a separator.
 >>> "n".join(["a","b"])
 'anb'
It results in a defconfig that does not end in a newline.

When BR2_JLEVEL is added by -j logic to the defconfig it ends up
concatenated to the last line of the defconfig.
 BR2_TOOLCHAIN_EXTERNAL_CODESOURCERY_ARM=yBR2_JLEVEL=7
The resulting .config has the default BR2_JLEVEL=0.

Instead of just workaround this problem by adding a newline before
BR2_JLEVEL when -j is used, make the defconfig to end in a newline since
it is a more future-proof solution.

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