[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.

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+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.
+
 2020-03-12  Tom de Vries  <tdevries@suse.de>
 
 	* lib/gdb.exp (gdb_core_cmd): Use string_to_regexp for regexp-matching

gdb/testsuite/gdb.linespec/explicit.exp

@@ -239,8 +239,26 @@ namespace eval $testfile {
 
 	set tst "complete non-unique file name"
 	send_gdb "break -source exp\t"
+	# We're matching two cases here:
+	# - without GLIBC debuginfo
+	#   (gdb) break -source exp^Glicit^G^M
+	#   explicit.c  explicit2.c  ^M
+	#   (gdb) break -source explicit^M
+	#   Source filename requires function, label, or line offset.^M
+	#   (gdb) PASS: gdb.linespec/explicit.exp: complete non-unique file name
+	# - with GLIBC debuginfo:
+	#   (gdb) break -source exp^Gl^G^M
+	#   explicit.c  explicit2.c  explicit_bzero.c  explicit_bzero_chk.c \
+	#     explodename.c  ^M
+	#   (gdb) break -source expl^M
+	#   Source filename requires function, label, or line offset.^M
+	#   (gdb) PASS: gdb.linespec/explicit.exp: complete non-unique file name
 	gdb_test_multiple "" $tst {
-	    -re "break -source exp\\\x07licit" {
+	    -re "break -source exp\\\x07l" {
+		# At this point, either output is done (first case), or a
+		# further "icit" is emitted (second case).  We have no reliable
+		# way to decide one way or another, so just send the tabs, even
+		# though that may be a little early in the second case.
 		send_gdb "\t\t"
 		gdb_test_multiple "" $tst {
 		    -re "\\\x07\r\nexplicit.c\[ \t\]+explicit2.c\[ \t\]+\(expl.*\)?\r\n$gdb_prompt" {