Fix the problem of not enough merged dtb size

The dtc has suffix compression for string, ex. "a_string", "string"
and "ing", these strings could share a same string in dtb
string table. libufdt assumes the merged dtb size shouldn't larger
than the summary size of base and overlay dtb, and doesn't
apply the same compression.

The patch add a test case for this case and fix the problem.

Bug: 35255584
Test: tests/run_tests.sh
Change-Id: I6bac1e2823ca90dbdb852452544c41ab040a5ccc