sac tests: do not use filecmp.

Output SAC files are not guranteed to be byte-by-byte identical to input
SAC files after reading and writing, without any other operation.

This because some header fields (npts, depmin, depmen, depmax, scale)
can be changed on writing to fix uncorrect/unconsistent values.

The use of byte-by-byte comparison filecmp is replaced by separate check
of several header fields and of the data arrays.