cgraph: add selftest::symbol_table_test

cgraph: add selftest::symbol_table_test

This patch adds a selftest fixture for overriding the "symtab" global,
so that selftests involving symtab nodes can be isolated from each
other: each selftest can have its own symbol_table instance.

In particular, this ensures that nodes can have a predictable "order"
and thus predictable dump names within selftests.

gcc/ChangeLog:
	* cgraph.c: Include "selftest.h".
	(saved_symtab): New variable.
	(selftest::symbol_table_test::symbol_table_test): New ctor.
	(selftest::symbol_table_test::~symbol_table_test): New dtor.
	(selftest::test_symbol_table_test): New test.
	(selftest::cgraph_c_tests): New.
	* cgraph.h (saved_symtab): New decl.
	(selftest::symbol_table_test): New class.
	* selftest-run-tests.c (selftest::run_tests): Call
	selftest::cgraph_c_tests.
	* selftest.h (selftest::cgraph_c_tests): New decl.

From-SVN: r265915