support/testing: TestXvisor: bump custom opensbi version to 1.6

Since Buildroot commit [1] "update to Bootlin toolchains 2025.08-1",
the TestXvisor fails when building xvisor with gcc 15.1.0.

Since xvisor is now fixed, opensbi 1.5 fail to build with gcc-15 for
the same reasons.

opensbi 1.6 include a fix for C23 enabled compilers [2]

[1] https://gitlab.com/buildroot.org/buildroot/-/commit/947dbc92a20c5acea7882166cae7893e6ea661e1
[2] https://github.com/riscv-software-src/opensbi/commit/d539d64a23bbd64ba324eed595449bcc9c7be0cb

Cc: Julien Olivain <ju.o@free.fr>
Signed-off-by: Romain Naour <romain.naour@smile.fr>
Signed-off-by: Julien Olivain <ju.o@free.fr>

support/testing/tests/package/test_xvisor.py

@@ -16,7 +16,7 @@ class TestXvisor(infra.basetest.BRTest):
         # BR2_TARGET_ROOTFS_TAR is not set
         BR2_TARGET_OPENSBI=y
         BR2_TARGET_OPENSBI_CUSTOM_VERSION=y
-        BR2_TARGET_OPENSBI_CUSTOM_VERSION_VALUE="1.5"
+        BR2_TARGET_OPENSBI_CUSTOM_VERSION_VALUE="1.6"
         BR2_TARGET_OPENSBI_PLAT="generic"
         BR2_PACKAGE_HOST_QEMU=y
         BR2_PACKAGE_HOST_QEMU_SYSTEM_MODE=y