subsys/testsuite: use bool for condition types

Use of the test suite in C++ causes warnings because use of defined
cast operators have the wrong target type.  For example, many standard
container APIs use operator bool() to test for empty containers.  Code
like zassert_true(v, "") fails to build when the test parameter is an
int.  Correct the argument type.

This also causes any use of an assignment expression as a conditional
in zassert to be diagnosed as a potential error.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>