Add test for string evaluation with "debug expression" on

As Joel pointed out in...
https://sourceware.org/ml/gdb-patches/2014-07/msg00391.html
...it would be nice to add a test for that.

Tested on Linux x86_64 (Ubuntu 14.10).

gdb/testsuite/ChangeLog

2014-07-15  Simon Marchi  <simon.marchi@ericsson.com>

	* gdb.base/debug-expr.exp: Test string evaluation with
	"debug expression" on.

gdb/testsuite/ChangeLog

+2014-07-15  Simon Marchi  <simon.marchi@ericsson.com>
+
+	* gdb.base/debug-expr.exp: Test string evaluation with
+	"debug expression" on.
+
 2014-07-15  Pedro Alves  <palves@redhat.com>
 
 	* gdb.base/reread.exp: Use clean_restart.

gdb/testsuite/gdb.base/debug-expr.exp

@@ -50,3 +50,8 @@ gdb_test_no_output "set debug expression 1"
 # This caused gdb to segfault.
 test_debug_expr "print /x {char\[4\]} array" \
     "\[$\]$decimal = \\{0x0, 0x1, 0x2, 0x3\\}"
+
+# This caused gdb to output garbage and possibly segfault
+gdb_test "print \"hello\"" \
+    ".*OP_STRING\[^\r\n\]*Language-specific string type: 0.*\[\r\n\]\\$$decimal = \"hello\"\[\r\n\].*" \
+    "String evaluation with debug expr"