[gdb/testsuite] Fix check-read1 FAIL in attach-many-short-lived-threads.exp

When running test-case gdb.threads/attach-many-short-lived-threads.exp with
check-read1, I ran into:
...
FAIL: gdb.threads/attach-many-short-lived-threads.exp: iter 1: \
  no new threads (timeout)
...

Fix this by rewriting the gdb_test_multiple call using -lbl and exp_continue.

Tested on x86_64-linux, with make targets check and check-read1.

gdb/testsuite/ChangeLog:

2020-03-14  Tom de Vries  <tdevries@suse.de>

	* gdb.threads/attach-many-short-lived-threads.exp: Read "info threads"
	result in line-by-line fashion.

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2020-03-14  Tom de Vries  <tdevries@suse.de>
+
+	* gdb.threads/attach-many-short-lived-threads.exp: Read "info threads"
+	result in line-by-line fashion.
+
 2020-03-14  Tom de Vries  <tdevries@suse.de>
 
 	* lib/gdb.exp (supports_statement_frontiers): New proc.

gdb/testsuite/gdb.threads/attach-many-short-lived-threads.exp

@@ -108,12 +108,18 @@ proc test {} {
 	    sleep 1
 
 	    set test "no new threads"
-	    gdb_test_multiple "info threads" $test {
-		-re "New .*$gdb_prompt $" {
-		    fail $test
+	    set status 1
+	    gdb_test_multiple "info threads" $test -lbl {
+		-re "\r\n\[^\r\n\]*New " {
+		    set status 0
+		    exp_continue
 		}
-		-re "$gdb_prompt $" {
-		    pass $test
+		-re -wrap "" {
+		    if { $status == 1 } {
+			pass $gdb_test_name
+		    } else {
+			fail $gdb_test_name
+		    }
 		}
 	    }