gdb/testsuite: Remove duplicates from gdb.base/dfp-exprs.exp

When I run the testsuite, I have:

    Running ../gdb/testsuite/gdb.base/dfp-exprs.exp ...
    DUPLICATE: gdb.base/dfp-exprs.exp: p 1.2dl < 1.3df

Replace hand-written tests checking various comparison operators between
various decimal floating point types with a loop to programmatically
generate all the combinations.  This removes the need to eyeball for all
suffixes, which lead to the original duplication.

Also add a lot more combinations, testing all comparison operators
comprehensively.  The result is 262 unique tests vs 104 before this
patch.

Tested on x86_86-linux.

Change-Id: Id215a3d610aa8e032bf06ee160b5e3aed4a92d1e