package/zxing-cpp: disable testrunner

package/zxing-cpp: disable testrunner

In case cppunit dependency is found the testrunner is build
which needs c++11 compile support enabled, avoid by building
only libzxing and zxing targets.

Fixes:
  - http://autobuild.buildroot.net/results/f7c2c03a2b5a0147a041d873c1a36143861be764

  [ 85%] Building CXX object CMakeFiles/testrunner.dir/core/tests/src/common/BitArrayTest.cpp.o
  In file included from .../host/opt/ext-toolchain/mips64el-buildroot-linux-uclibc/include/c++/5.5.0/type_traits:35:0,
                   from .../host/mips64el-buildroot-linux-uclibc/sysroot/usr/include/cppunit/tools/StringHelper.h:7,
                   from .../host/mips64el-buildroot-linux-uclibc/sysroot/usr/include/cppunit/TestAssert.h:8,
                   from .../host/mips64el-buildroot-linux-uclibc/sysroot/usr/include/cppunit/TestCase.h:6,
                   from .../host/mips64el-buildroot-linux-uclibc/sysroot/usr/include/cppunit/TestCaller.h:5,
                   from .../host/mips64el-buildroot-linux-uclibc/sysroot/usr/include/cppunit/extensions/HelperMacros.h:9,
                   from .../build/zxing-cpp-0db7f855135222becff193671faae79c083424b6/core/tests/src/common/BitArrayTest.h:24,
                   from .../build/zxing-cpp-0db7f855135222becff193671faae79c083424b6/core/tests/src/common/BitArrayTest.cpp:21:
  .../host/opt/ext-toolchain/mips64el-buildroot-linux-uclibc/include/c++/5.5.0/bits/c++0x_warning.h:32:2: error: #error This file requires compiler and library support for the ISO C++ 2011 standard. This support must be enabled with the -std=c++11 or -std=gnu++11 compiler options.

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>