test: verify filesystem sizes when boot testing

During the boot test (test_image_boots()), run 'df' in the VM and verify
that the partitions and filesystems on disk match the filesystem
customizations from the build config.

Previously, we only had a check for /sysroot, but that wasn't doing
anything because there was no call to 'df' preceding it.  The current
test first checks that every mountpoint specified in the build config
exists in the VM and that the minsizes are respected (with some offsets
that will be investigated separately).

test/test_build.py

@@ -355,14 +355,37 @@ def test_image_boots(image_type):
         # XXX: read the fully yaml instead?
         assert f"image: {image_type.container_ref}" in output
 
-        # Figure out how big / is and make sure it is > 11bGiB
-        # Note that df output is in 1k blocks, not bytes
-        for line in output.splitlines():
+        # check the minsize specified in the build configuration for each mountpoint against the sizes in the image
+        exit_status, output = test_vm.run("df --output=target,size", user="root",
+                                          keyfile=image_type.ssh_keyfile_private_path)
+        assert exit_status == 0
+        # parse the output of 'df' to a mountpoint -> size dict for convenience
+        mountpoint_sizes = {}
+        for line in output.splitlines()[1:]:
             fields = line.split()
-            if fields[0] == "/sysroot":
-                size = int(fields[1])
-                assert size > 11 * 1024 * 1024
-                break
+            # Note that df output is in 1k blocks, not bytes
+            mountpoint_sizes[fields[0]] = int(fields[1]) * 2 ** 10  # in bytes
+
+        fs_customizations = testutil.create_filesystem_customizations(image_type.rootfs)
+        for fs in fs_customizations:
+            mountpoint = fs["mountpoint"]
+            if mountpoint == "/":
+                # / is actually /sysroot
+                mountpoint = "/sysroot"
+            assert mountpoint in mountpoint_sizes
+
+            minsize_human = fs["minsize"]
+            # assume all sizes are GiB
+            minsize_str = minsize_human.removesuffix("GiB").strip()
+            minsize = int(minsize_str) * 2 ** 30
+            # TODO: find the exact source of all the discrepancies or compare the actual partition sizes instead of the
+            # filesystem sizes
+            if mountpoint == "/sysroot":
+                minsize -= 2 ** 30  # reduce expected /sysroot size by 1 GiB
+
+            # NOTE: xfs filesystems are ~40 MiB and ext4 ~26 MiB smaller than the partition - reduce minsize by 100 MiB
+            minsize -= 100 * 2 ** 20
+            assert minsize < mountpoint_sizes[mountpoint]
 
 
 @pytest.mark.parametrize("image_type", gen_testcases("ami-boot"), indirect=["image_type"])