[gdb/testsuite] Extend gdb.base/index-cache.exp further

Add lookup of a non-existing symbol to test-case gdb.base/index-cache.exp.

This serves as regression test for PR symtab/30718.

PR symtab/30718
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30718

gdb/testsuite/gdb.base/index-cache.exp

diff --git a/gdb/testsuite/gdb.base/index-cache.exp b/gdb/testsuite/gdb.base/index-cache.exp
index 1379a04..22f7a54 100644
--- a/gdb/testsuite/gdb.base/index-cache.exp
+++ b/gdb/testsuite/gdb.base/index-cache.exp
@@ -152,6 +152,9 @@
 	    # Trigger expansion of symtab not containing main.
 	    gdb_test "ptype foo" "^type = int \\(void\\)"
 
+	    # Look for non-existent function.
+	    gdb_test "ptype foobar" "^No symbol \"foobar\" in current context\\."
+
 	    check_cache_stats 0 0
 	}
     }
@@ -199,6 +202,9 @@
 	# Trigger expansion of symtab not containing main.
 	gdb_test "ptype foo" "^type = int \\(void\\)"
 
+	# Look for non-existent function.
+	gdb_test "ptype foobar" "^No symbol \"foobar\" in current context\\."
+
 	if { $expecting_index_cache_use } {
 	    check_cache_stats 0 1
 	} else {
@@ -234,6 +240,9 @@
 	# Trigger expansion of symtab not containing main.
 	gdb_test "ptype foo" "^type = int \\(void\\)"
 
+	# Look for non-existent function.
+	gdb_test "ptype foobar" "^No symbol \"foobar\" in current context\\."
+
 	if { $expecting_index_cache_use } {
 	    check_cache_stats 1 0
 	} else {