Eliminate code duplication in all.tests for depthstencil-render-miplevels

No functional change.
depthstencil-render-miplevels: Add test for size 273.

When a mipmapped depth texture's size satisfies size > 1 and (size %
16) == 1, this causes miplevel 1 to have a size that is a multiple of
8, but not to be aligned on a multiple of 8 boundary.  This turns out
to be an important corner case for the i965 driver.

Note: in an ideal world, I'd modify depthstencil-render-miplevels to
simply test every possible buffer size from 1 to some reasonably large
size (perhaps 256), but this would produce an impractically large
number of test cases.  So instead I'm just testing the sizes that have
been known to cause problems.