Test: add return value for mock in tests to suppress compile warning

[Cstandardlib]

Linked Issue

Fix #4756

What's changed?

• Add missing return value for mock in some tests to suppress compile warnings.
• The relevant parts of gtest are left unchanged.

// like this
abacus-develop/source/module_elecstate/module_dm/test/test_cal_dm_R.cpp:77:10: warning: 'TearDown' overrides a member function but is not marked 'override' [-Winconsistent-missing-override]
   77 |     void TearDown()
      |          ^
/usr/local/include/gtest/gtest.h:315:16: note: overridden virtual function is here
  315 |   virtual void TearDown();
      |                ^