test/formatter: Add tests for dump_format_ns

test/formatter: Add tests for dump_format_ns

Adding basic unit test to test the new formatter class'
dump_format_ns. Since the functionality only affects XML (and other
implementations mimic dump_format exactly), tests are added for these.
`fmt.close_section()` is avoided in the tests as this calls an
assert (and there is no section to close) and this triggers a test
failure.

Signed-off-by: Abhishek Lekshmanan <abhishek.lekshmanan@ril.com>