tests/Makefile: Restrict some softmmu-only tests

tests/Makefile: Restrict some softmmu-only tests

In the next commit we are going to remove some objects from the
util-obj-y variable (objects which are not used by user-mode,
when configured with --disable-system).
Then some system-mode tests are going to fail, due to the missing
objects:

  $ make check-unit -k
    LINK    tests/test-iov
  /usr/bin/ld: tests/test-iov.o: in function `iov_from_buf':
  include/qemu/iov.h:49: undefined reference to `iov_from_buf_full'
  make: *** [rules.mak:124: tests/test-iov] Error 1
    LINK    tests/test-timed-average
  /usr/bin/ld: tests/test-timed-average.o: in function `account':
  tests/test-timed-average.c:27: undefined reference to `timed_average_account'
  make: *** [rules.mak:124: tests/test-timed-average] Error 1
    LINK    tests/test-util-filemonitor
  /usr/bin/ld: tests/test-util-filemonitor.o: in function `qemu_file_monitor_test_event_loop':
  tests/test-util-filemonitor.c:83: undefined reference to `main_loop_wait'
  make: *** [rules.mak:124: tests/test-util-filemonitor] Error 1
    LINK    tests/test-util-sockets
  /usr/bin/ld: tests/test-util-sockets.o: in function `test_socket_fd_pass_name_good':
  tests/test-util-sockets.c:91: undefined reference to `socket_connect'
  make: *** [rules.mak:124: tests/test-util-sockets] Error 1
    LINK    tests/test-base64
  /usr/bin/ld: tests/test-base64.o: in function `test_base64_good':
  tests/test-base64.c:35: undefined reference to `qbase64_decode'
  collect2: error: ld returned 1 exit status
  make: *** [rules.mak:124: tests/test-base64] Error 1
    LINK    tests/test-bufferiszero
  /usr/bin/ld: tests/test-bufferiszero.o: in function `test_1':
  tests/test-bufferiszero.c:31: undefined reference to `buffer_is_zero'
  make: *** [rules.mak:124: tests/test-bufferiszero] Error 1
  make: Target 'check-unit' not remade because of errors.

Instead, restrict these tests to system-mode, by using the
$(CONFIG_SOFTMMU) variable.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Tested-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200522172510.25784-5-philmd@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>