subsys/cpp: Use toolchain picolibc for C++ tests

The picolibc module is not supported for C++, so make sure
the toolchain has C++ support and then use it for the tests.

Signed-off-by: Keith Packard <keithp@keithp.com>

Author: keith-packard

tests/subsys/cpp/cxx/testcase.yaml

@@ -22,6 +22,6 @@ tests:
       - CONFIG_NEWLIB_LIBC_NANO=y
   cpp.main.picolibc:
     tags: picolibc
-    filter: CONFIG_PICOLIBC_SUPPORTED
+    filter: (TOOLCHAIN_HAS_PICOLIBC == 1) and CONFIG_PICOLIBC_SUPPORTED
     extra_configs:
       - CONFIG_PICOLIBC=y