PATCH: Use runtest_file_p in testsuite/runtest.libs/libs.exp

This is a small change to make quickly running a few of the internal 
library tests easier.  This was added to ease development of improved 
tests for lib/target.exp.
With this change, "$srcdir/runtest --tool runtest --srcdir $srcdir 
libs.exp=target.test" runs only the tests in target.test; previously 
there was no easy way to select a subset of the library tests.

----
ChangeLog entry:
    	* testsuite/runtest.libs/libs.exp: Use runtest_file_p to honor
    	requests to run only some library tests.
---
diff --git a/testsuite/runtest.libs/libs.exp b/testsuite/runtest.libs/libs.exp
index 8521ead..f42d3db 100644
--- a/testsuite/runtest.libs/libs.exp
+++ b/testsuite/runtest.libs/libs.exp
@@ -1,4 +1,4 @@
-# Copyright (C) 1997-2016, 2018 Free Software Foundation, Inc.
+# Copyright (C) 1997-2016, 2018, 2019 Free Software Foundation, Inc.
 #
 # This file is part of DejaGnu.
 #
@@ -98,7 +98,7 @@ proc process_test { test } {
 
 start_expect
 foreach i [glob [testsuite file -source -test *.test]] {
-    process_test $i
+    if { [runtest_file_p $runtests $i] } { process_test $i }
 }
 stop_expect
 
----

-- Jacob