binman: ftest: pass allow_fake_blob to _DoReadFileDtb

binman: ftest: pass allow_fake_blob to _DoReadFileDtb

Some test cases don't use _DoTestFile directly which accepts
allow_fake_blobs. However, they specifically test functionality that
requires external blobs not to be faked. Extend the _DoReadFileDtb
signature to allow passing that option to _DoTestFile.

Also fix tests that require non-faked ext blobs.
By default, external blobs are faked. Some tests care only about more
basic functionality. In those cases no external blobs should be faked.
That would trigger a different (binman) case which is not in scope for
those particular tests.
Thus, disable faked blobs for those test cases.

Signed-off-by: Yannic Moog <y.moog@phytec.de>
Reviewed-by: Bryan Brattlof <bb@ti.com>