[gdb/testsuite] Fix unbalanced braces in gdb.tui/new-layout.exp

On SLE-11 with tcl version 8.5, we run into:
...
UNRESOLVED: gdb.tui/new-layout.exp: testcase aborted due to \
  invalid command name: }
ERROR: Couldn't send tui new-layout example src 1} to GDB.
...

Apparently tcl 8.6 is more forgiving about unbalanced braces.

Expose the unbalanced braces on tcl 8.6 by temporarily wrapping the
test-case in:
...
proc do_this { args } {
    uplevel 1 {*}$args
}

do_this {
  ...
}
...
and escape them using a backslash.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-09-16  Tom de Vries  <tdevries@suse.de>

	PR testsuite/26618
	* gdb.tui/new-layout.exp: Escape unbalanced braces.