oniro-image-base-tests: bump rootfs size to 2500M

Building the base test image with debug mode enabled returns the below
error.

```
ERROR: File system image of partition / is larger (2269180 kB) than its
allowed size 2097152 kB.
```

Increasing rootfs size to 2500M solves the issue.

Signed-off-by: Chase Qi <chase.qi@linaro.org>

meta-oniro-core/recipes-core/images/oniro-image-base-tests.bb

@@ -19,3 +19,5 @@ EXTRA_IMAGE_FEATURES:append = " ptest-pkgs"
 # For testing purposes, we want to have an extra ext4 format for the root
 # filesystem.
 IMAGE_FSTYPES:append = " ext4"
+
+ROOT_PARTITION_SIZE = "2500M"