Fix TUI support checks in gdb.tui tests.

Fix TUI support checks in gdb.tui tests.

2020-06-17 Sandra Loosemore <sandra@codesourcery.com>

	gdb/testsuite/
	* gdb.tui/basic.exp: Skip test when TUI is unsupported, don't
	just say UNSUPPORTED.
	* gdb.tui/corefile-run.exp: Likewise.
	* gdb.tui/empty.exp: Likewise.
	* gdb.tui/list-before.exp: Likewise.
	* gdb.tui/list.exp: Likewise.
	* gdb.tui/main.exp: Likewise.
	* gdb.tui/regs.exp: Likewise.
	* gdb.tui/resize.exp: Likewise.
	* gdb.tui/tui-layout-asm-short-prog.exp: Likewise.
	* gdb.tui/tui-layout-asm.exp: Likewise.
	* gdb.tui/tui-missing-src.exp: Likewise.
	* gdb.tui/winheight.exp: Likewise.
	* gdb.tui/new-layout.exp: Likewise.  Also move check earlier.