[gdb/testsuite] Fix gdb.tui/new-layout.exp with tcl 8.5

[gdb/testsuite] Fix gdb.tui/new-layout.exp with tcl 8.5

In commit 4d91ddd342 "[gdb/testsuite] Fix unbalanced braces in
gdb.tui/new-layout.exp", I tried to fix a problem with test-case
gdb.tui/new-layout.exp when running with tcl 8.5.

However, at that point I only had access to the log containing the failure,
and unfortunately my patch turned out not to be effective.

So, finally fix this problem by guarding the problematic code with:
...
if { [tcl_version_at_least 8 6] } { ... }
...

Tested on x86_64-linux, specifically SLE-11 where I ran into the failure.

gdb/testsuite/ChangeLog:

2020-12-10  Tom de Vries  <tdevries@suse.de>

	PR testsuite/26947
	* gdb.tui/new-layout.exp: Don't execute tests with unbalanced curly
	braces for tcl 8.5 and earlier.