Set an abs_zero for the adapt_and_modify test.
Set an abs_zero for the adapt_and_modify test.

This test diffed ever so slightly on our new 64-bit build target (see
below) so it might be possible to fix simply by loosening the abs_zero
setting slightly.

Refs #6959.

   --------- Time step 1, 4.0000000e-03 ~ 4.0000000e-03, rel diff:  0.00000e+00 ---------
 Nodal variables:
    u  rel diff:  1.5019635e-10 ~  1.4481588e-10 = 3.58229e-02 (node 230)
 Element variables:
    gji    rel diff:  5.4255747e-09 ~  5.4316172e-09 = 1.11246e-03 (block 0, elmt 214)

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