tests/subsys/llext: fix coverage for readonly_mmu test

The test was not being run on arm64 and riscv because the filter was
incorrect. Properly fix the filter to run the test on all platforms
that have any kind of MMU enabled.

Signed-off-by: Luca Burelli <[email protected]>

Author: pillo79

tests/subsys/llext/simple/testcase.yaml

@@ -50,10 +50,11 @@ tests:
       - CONFIG_LLEXT_STORAGE_WRITABLE=n
   llext.simple.readonly_mmu:
     arch_allow: arm64 arm riscv
-    filter: CONFIG_ARM_MMU
+    filter: CONFIG_MMU or CONFIG_RISCV_PMP
     integration_platforms:
       - qemu_cortex_a53         # ARM Cortex-A53 (ARMv8-A ISA)
     extra_configs:
+      - CONFIG_LLEXT_HEAP_SIZE=128 # qemu_cortex_a9 requires larger heap
       - CONFIG_LLEXT_STORAGE_WRITABLE=n
   llext.simple.writable:
     arch_allow: arm xtensa riscv