subsys/testsuite/coverage: move initialization vector to RODATA

The .init_array vector is, in fact, read-only data, so put it there.

Previously it was placed at the end of the ROM, but was unknown to
the x86 memory protection code with XIP enabled (because it was not
part of the text, rodata, or kernel RAM). Until recently, the XIP
implementation artificially bloated _image_rodata_size to cover the
entire ROM, so the (mis)placement of .init_array went unnoticed.

Signed-off-by: Charles E. Youse <charles.youse@intel.com>