TCF: disable running single core testcases on Quark SE's x86+arc

By default TCF was running test single core test cases in both Quark
SE cores at the same time.

This worked because the ARC output came through the IPM console to
x86, which enabled them to coexist.

However, after default configuration changes, this is no longer
possible without adding extra hardware to the current test setup,
which is cost prohibitive, and thus we are driving ARC output to the
same port as the x86 output.

This would make the output single-core test cases that are ran at the
same time in the x86 and ARC cores mix their outputs.

So we have that scenario disabled (and makes us run less test
cases). However, multicore test cases still can be run by selecting:

@targets bsp_model:x86+arc

Change-Id: I5f6b43246a24d0b263fd4a47866c7f0f99d20505
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>