Add "style" proc to the test suite

Add "style" proc to the test suite

This adds a "style" helper proc to the test suite, and updates
existing style tests to use it.  Thanks to Sergio for the idea.

Tested on x86-64 Fedora 29.

gdb/testsuite/ChangeLog
2019-05-22  Tom Tromey  <tromey@adacore.com>

	* gdb.base/info-shared.exp (check_info_shared): Use "style".
	* gdb.base/style.exp: Use "style".
	* lib/gdb-utils.exp (style): New proc.