test: allow SKIP_INITRD override

Certain distributions (e.g. Arch Linux) require booting with initrd, as
they lack support for commonly used filesystems in the kernel (i.e. the
support is compiled in as modules)

test/test-functions

@@ -50,7 +50,7 @@ IS_BUILT_WITH_ASAN=$(is_built_with_asan && echo yes || echo no)
 
 if [[ "$IS_BUILT_WITH_ASAN" = "yes" ]]; then
     STRIP_BINARIES=no
-    SKIP_INITRD=yes
+    SKIP_INITRD="${SKIP_INITRD:-yes}"
     PATH_TO_INIT=$ROOTLIBDIR/systemd-under-asan
     QEMU_MEM="1536M"
     QEMU_SMP=4