test-configs: conditional QEMU command-line args based on Kconfig

When generating tests, would like to add QEMU command-line arguments that are dependent on kernel Kconfig options.

For example, add the `-smp` argument to QEMU only if the kernel has `CONFIG_SMP=y` in the defconfig.

Or maybe easier would be to use existing defconfig filters and only add `-smp` to the command-line if `CONFIG_SMP=n` is *not* in the defconfig filter.

No file or test is named. Start by tracing test generation and the existing defconfig-filter handling, then verify generated QEMU command lines for configurations with and without CONFIG_SMP; done means conditional arguments are reflected in generated tests.

Written by the indexing model from the issue text.