gdb/testsuite: Use 'array unset' instead of just 'unset'

gdb/testsuite: Use 'array unset' instead of just 'unset'

In the check-test-names.exp library 'unset' was being used to unset an
array variable.  Though this seems to work fine on tcl 8.6, it was
discovered on a CentOS 7.8.2003 machine, running tcl 8.5, that this
doesn't work and 'array unset' should be used instead.

Using 'array unset' should work fine for newer and older versions of
tcl (since 8.3, releases ~2000).

gdb/testsuite/ChangeLog:

	* lib/check-test-names.exp (do_reset_vars): Use 'array unset' to
	unset the array variable.