gdb/testsuite: new test for -data-disassemble opcodes format

gdb/testsuite: new test for -data-disassemble opcodes format

Add another test for the output of MI command -data-disassemble.  The
new check validates the format of the 'opcodes' field, specifically,
this test checks that the field contains a series of bytes, separated
by a single space.

We also check that the bytes are in the correct order, that is, the
first byte is from the lowest address, and subsequent bytes are from
increasing addresses.

The motivation for this test (besides more tests being generally good)
is that I plan to make changes to how opcode bytes are displayed in
the disassembler output, and I want to ensure that I don't break any
existing MI behaviour.

There should be no user visible changes to GDB after this commit.