[gdb/testsuite] Fix gdb.base/sect-cmd.exp

[gdb/testsuite] Fix gdb.base/sect-cmd.exp

With a testsuite setup modified to make expect wait a little bit longer for
gdb output (see PR27957), I reliably run into:
...
(gdb) FAIL: gdb.base/sect-cmd.exp: set section .text to original \
  address (timeout)
...

The problem is a too greedy regexp:
...
       -re ".*$address1 \- $address2 is $section_name.*" {
...
which ends up consuming the gdb prompt with the terminating ".*".

Fix this by limiting the regexp to a single line.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2021-06-08  Tom de Vries  <tdevries@suse.de>

	* gdb.base/sect-cmd.exp: Fix saw_section_address_line regexp.