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.