tests/tcg: use CONFIG_LINUX_USER, not CONFIG_LINUX

tests/tcg: use CONFIG_LINUX_USER, not CONFIG_LINUX

The two more or less overlap, because CONFIG_LINUX is a requirement for Linux
user-mode emulation.  However, CONFIG_LINUX is technically a host symbol
that applies even to system emulation.  Defining CONFIG_LINUX_USER, and
CONFIG_BSD_USER for eventual future use, is cleaner.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20211210084836.25202-1-pbonzini@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20220105135009.1584676-20-alex.bennee@linaro.org>