[gdb/testsuite] Fix gdb.linespec/explicit.exp FAIL with glibc debug info

[gdb/testsuite] Fix gdb.linespec/explicit.exp FAIL with glibc debug info

When running test-case gdb.linespec/explicit.exp with GLIBC debuginfo
installed, I run into:
...
(gdb) break -source exp^GlFAIL: gdb.linespec/explicit.exp: complete \
  non-unique file name (timeout)
...

The regexp that times out is:
...
           -re "break -source exp\\\x07licit" {
...
and the reason it times out is that gdb only outputs an "l" after the tab, while
the regexp expect a futher "icit".

This is a regression since commit 507dd60e28 "[gdb/testsuite, 1/2] Fix
gdb.linespec/explicit.exp with check-read1", where I merged the matching for
the two cases where GLIBC debuginfo is either installed or not, as it turns
out incorrectly, presumably because even though I tested with GLIBC debuginfo
info installed and deinstalled, that didn't make a difference because I didn't
use configure flag --with-separate-debug-dir=/usr/lib/debug.

Fix this by not explictly matching the "icit" part.

Tested on x86_64-linux, with and without GLIBC debuginfo installed, both with
make targets check and check-read1.

gdb/testsuite/ChangeLog:

2020-03-12  Tom de Vries  <tdevries@suse.de>

	* gdb.linespec/explicit.exp: Fix "complete non-unique file name" test
	in presence of GLIBC debuginfo.