Tests: Bluetooth: Tester: Mesh DFUM and BLOB should have own elements

Moving the blob client, dfd server and dfu server to their own
elements and increasing the CONFIG_BT_MESH_TX_SEG_MAX to 8 to fit
new composition data into composition data page status message.

Standalone mesh blob client and DFU distributor/update server models
requires one element and those elements only contain the main models
and the models they extend to.

Referring to MshMBT_v1.0 Section 6.1:
The BLOB Transfer Client model defines the messages listed in Table 6.1
, and requires one element: the BLOB Transfer Client Main element. The
BLOB Transfer Client Main element contains the BLOB Transfer Client
main model.

And referring to MshDFU_v1.0 Sections 6.1.1 and 6.2.1:
6.1.1
The Firmware Update Server model adds the state instances listed in
Table 6.1 and Table 6.2 and the messages listed in Table 6.3 to the
model it extends, and requires one element: the Firmware Update Main
element. The Firmware Update Main element contains the Firmware Update
Server main model and all the models that the main model extends.
6.2.1
The Firmware Distribution Server model adds the state instances listed
in Table 6.7 and Table 6.8 and the messages listed in Table 6.9 to the
model it extends, and requires one element: the Firmware Distribution
Main element. The Firmware Distribution Main element contains the
Firmware Distribution Server main model and all the models that the
main model extends.

Signed-off-by: alperen sener <alperen.sener@nordicsemi.no>