gdb/testsuite: Remove duplicates from gdb.base/pie-fork.exp

When running the testsuite, I have:

    Running .../gdb/testsuite/gdb.base/pie-fork.exp ...
    DUPLICATE: gdb.base/pie-fork.exp: test_no_detach_on_fork: continue

Fix by giving explicit names to the 'continue' commands that cause the
duplicate message.

Tested on x86_64-linux.
diff --git a/gdb/testsuite/gdb.base/pie-fork.exp b/gdb/testsuite/gdb.base/pie-fork.exp
index a60e420..efc357d 100644
--- a/gdb/testsuite/gdb.base/pie-fork.exp
+++ b/gdb/testsuite/gdb.base/pie-fork.exp
@@ -54,9 +54,11 @@
 proc_with_prefix test_no_detach_on_fork {} {
     setup_test "off"
 
-    gdb_test "continue" "\r\nThread 1.1 .* hit Breakpoint 2, break_here.*"
+    gdb_test "continue" "\r\nThread 1.1 .* hit Breakpoint 2, break_here.*" \
+	     "continue from thread 1.1"
     gdb_test "thread 2.1"
-    gdb_test "continue" "\r\nThread 2.1 .* hit Breakpoint 2, break_here.*"
+    gdb_test "continue" "\r\nThread 2.1 .* hit Breakpoint 2, break_here.*" \
+	     "continue from thread 2.1"
 }
 
 test_detach_on_fork_follow_parent