tests: libcxx: Add LLVM libc++/picolibc test

This commit adds a new C++ standard library test variant that builds with
Picolibc and LLVM C++ Standard Library aka. libc++ on the supported
toolchains (e.g. LLVM Embedded Toolchain for Arm and Zephyr SDK LLVM).

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>

Author: stephanosio

tests/lib/cpp/libcxx/testcase.yaml

@@ -3,6 +3,7 @@ common:
     # see #89839
     - rx
 tests:
+  # GNU C++ Standard Library (libstdc++)
   cpp.libcxx.glibcxx.newlib:
     filter: TOOLCHAIN_HAS_GLIBCXX == 1 and TOOLCHAIN_HAS_NEWLIB == 1
     toolchain_exclude: xcc
@@ -38,13 +39,27 @@ tests:
       - CONFIG_CPP_EXCEPTIONS=y
     integration_platforms:
       - mps2/an385
+  # LLVM C++ Standard Library (libc++)
+  cpp.libcxx.libcxx.picolibc:
+    filter: TOOLCHAIN_HAS_LIBCXX == 1 and TOOLCHAIN_HAS_PICOLIBC == 1
+    toolchain_exclude: xcc
+    tags: cpp
+    timeout: 60
+    extra_configs:
+      - CONFIG_PICOLIBC=y
+      - CONFIG_LIBCXX_LIBCPP=y
+      - CONFIG_CPP_EXCEPTIONS=y
+    integration_platforms:
+      - mps2/an385
+  # ARC MWDT C++ Standard Library
   cpp.libcxx.arcmwdtlib:
     toolchain_allow: arcmwdt
     min_flash: 54
     tags: cpp
     extra_configs:
       - CONFIG_ARCMWDT_LIBC=y
       - CONFIG_ARCMWDT_LIBCPP=y
+  # Host C++ Standard Library
   cpp.libcxx.host:
     arch_allow: posix
     tags: cpp