test: pr_cont_test.o depends on CONFIG_LOG=y

Compiling wandboard_defconfig with CONFIG_UT_LOG=y leads to a build error:

test/log/pr_cont_test.c: In function ‘log_test_pr_cont’:
test/log/pr_cont_test.c:28:14: error:
‘gd_t’ {aka ‘volatile struct global_data’} has no member named ‘log_fmt’
  log_fmt = gd->log_fmt;

We do not want to let CONFIG_UT_LOG depend on CONFIG_LOG=y because we have
tests for logging functions called with CONFIG_LOG=n.

Fix the build dependency.

Reported-by: Kever Yang <kever.yang@rock-chips.com>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>