gdb/testsuite: adjust gdb.python/flexible-array-member.exp expected pattern

gdb/testsuite: adjust gdb.python/flexible-array-member.exp expected pattern

The `Type.range ()` tests in gdb.python/flexible-array-member.exp pass
when the test is compiled with gcc 9 or later, but not with gcc 8 or
earlier:

    $ make check TESTS="gdb.python/flexible-array-member.exp" RUNTESTFLAGS="CC_FOR_TARGET='gcc-8'"

    python print(zs['items'].type.range())^M
    (0, 0)^M
    (gdb) FAIL: gdb.python/flexible-array-member.exp: python print(zs['items'].type.range())
    python print(zso['items'].type.range())^M
    (0, 0)^M
    (gdb) FAIL: gdb.python/flexible-array-member.exp: python print(zso['items'].type.range())

The value that we get for the upper bound of a flexible array member
declared with a "0" size is 0 with gcc <= 8 and is -1 for gcc >= 9.
This is due to different debug info.  For this member, gcc 8 does:

    0x000000d5:   DW_TAG_array_type
                    DW_AT_type [DW_FORM_ref4]       (0x00000034 "int")
                    DW_AT_sibling [DW_FORM_ref4]    (0x000000e4)

    0x000000de:     DW_TAG_subrange_type
                      DW_AT_type [DW_FORM_ref4]     (0x0000002d "long unsigned int")

For the same type, gcc 9 does:

    0x000000d5:   DW_TAG_array_type
                    DW_AT_type [DW_FORM_ref4]       (0x00000034 "int")
                    DW_AT_sibling [DW_FORM_ref4]    (0x000000e5)

    0x000000de:     DW_TAG_subrange_type
                      DW_AT_type [DW_FORM_ref4]     (0x0000002d "long unsigned int")
                      DW_AT_count [DW_FORM_data1]   (0x00)

Ideally, GDB would present a consistent and documented value for an
array member declared with size 0, regardless of how the debug info
looks like.  But for now, just change the test to accept the two
values, to get rid of the failure and make the test in sync

I also realized (by looking at the py-type.exp test) that calling the
fields method on an array type yields one field representing the "index"
of the array.  The type of that field is of type range
(gdb.TYPE_CODE_RANGE).  When calling `.range()` on that range type, it
yields the same range tuple as when calling `.range()` on the array type
itself.  For completeness, add some tests to access the range tuple
through that range type as well.

gdb/testsuite/ChangeLog:

	* gdb.python/flexible-array-member.exp: Adjust expected range
	value for member declared with 0 size.  Test accessing range
	tuple through range type.

Change-Id: Ie4e06d99fe9315527f04577888f48284d649ca4c