gdb/testsuite: cleanup in gdb.base/args.exp

The last few commits resolved the KFAILs in gdb.base/args.exp.  With
those out of the way we can clean up this test script a little.

In this commit I have:

  - Stopped passing 'nowarnings' flag when building the source file.
    I see no reason why this source should issue a warning,

  - Moved setup of GDBFLAGS into args_test proc, callers that passed a
    newline needed a small tweak, and also the matching code needs
    updating for newline handling, but I think this is nicer, the
    argument lists are now given just once,

  - Updated comment on args_test,

  - Updated other comments.

There should be no change in what is tested after this commit.

Approved-By: Tom Tromey <tom@tromey.com>