fix(dfpt): use mark_occ_mat_initialized in pw_data test

The test called a nonexistent Plus_U_Base::set_occ_mat_initialized(bool);
the actual API is mark_occ_mat_initialized(). Verified locally:
MODULE_DFPT_pw_data_test 6/6 passed.

Author: abacus_fixer

source/source_pw/module_dfpt/test/dfpt_pw_data_test.cpp

@@ -202,7 +202,7 @@ TEST_F(DFPT_PW_DataTest, DftuReservationProviderUsability)
     EXPECT_FALSE(data.u_active());
     ASSERT_NE(data.get_dftu(), nullptr);
 
-    dftu.set_occ_mat_initialized(true);
+    dftu.mark_occ_mat_initialized();
     EXPECT_TRUE(data.u_active());
 
     data.clean();