support/testing: TestPythonPy3SciPy: increase rootfs size to 250M

The rootfs generated TestPythonPy3SciPy by doesn't fit anymore in 120M.

Increase the rootfs size to 250M and checking the amount of space
available on the file system.

  Filesystem                Size      Used Available Use% Mounted on
  /dev/root               232.3M    134.5M     85.3M  61% /

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/5880448601

Signed-off-by: Romain Naour <romain.naour@smile.fr>
Reviewed-by: Antoine Coutant <antoine.coutant@smile.fr>
Signed-off-by: Romain Naour <romain.naour@smile.fr>

support/testing/tests/package/test_python_scipy.py

@@ -10,7 +10,7 @@ class TestPythonPy3SciPy(TestPythonPackageBase):
         BR2_PACKAGE_PYTHON3=y
         BR2_PACKAGE_PYTHON_SCIPY=y
         BR2_TARGET_ROOTFS_EXT2=y
-        BR2_TARGET_ROOTFS_EXT2_SIZE="120M"
+        BR2_TARGET_ROOTFS_EXT2_SIZE="250M"
         # BR2_TARGET_ROOTFS_TAR is not set
         """
     sample_scripts = ["tests/package/sample_python_scipy.py"]