Add tests for new alias default-args related commands and arguments.
    
    Test the new default-args behaviour and completion for the alias command.
    Note that gdb.base/default-args.exp is somewhat copied from
    with.exp (the test of the with command), while default-exp.c
    is a plain copy of with.c.
    
    gdb/testsuite/ChangeLog
    2020-06-22  Philippe Waroquiers  <philippe.waroquiers@skynet.be>
    
    	* gdb.base/default-args.exp: New test.
    	* gdb.base/default-args.c: New file.
    	* gdb.base/alias.exp: Update expected error msg for alias foo=bar.
    	* gdb.base/default.exp: Update to new help text.
    	* gdb.base/help.exp: Likewise.
    	* gdb.base/page.exp: Likewise.
    	* gdb.base/style.exp: Likewise.
    	* gdb.guile/guile.exp: Likewise.
    	* gdb.python/python.exp: Likewise.