tests: add blob-buffer overflow test

tests: add blob-buffer overflow test

The blob buffer has no limitation in place
to prevent buflen to exceed maximum size.

This commit adds a test to demonstrate how
a blob increases past the maximum allowd
size of 16MB. It continuously adds chunks
of 64KB and with the 255th one blob_add()
returns a valid attribute pointer but the
blob's buflen does not increase.

The test is used to demonstrate the
failure, which is fixed with a follow-up
commit.

Signed-off-by: Zefir Kurtisi <zefir.kurtisi@gmail.com>
[adjusted test case for cram usage]
Signed-off-by: Petr Štetiar <ynezz@true.cz>