test: strengthen assertions, add sorted SDC diff, and clean up tests

- Split oversized test files to stay under 5,000 lines per file:
  TestSdc.cc → TestSdcClasses.cc, TestSdcStaInit.cc, TestSdcStaDesign.cc
  TestSearchStaDesign.cc → TestSearchStaDesign.cc, TestSearchStaDesignB.cc
  TestLibertyStaBasics.cc → TestLibertyStaBasics.cc, TestLibertyStaBasicsB.cc
  TestNetwork.cc → TestNetwork.cc, TestNetworkB.cc
- Replace ~200+ (void) casts with proper EXPECT_* assertions across all
  C++ test files (dcalc, liberty, network, sdc, search, power, spice, util)
- Remove ~55 SUCCEED() and EXPECT_TRUE(true) no-op assertions
- Fix 6 load-only Tcl tests by adding diff_files verification with
  22 new .sdcok golden reference files
- Delete 7 orphan .ok files with no matching .tcl tests
- Add how_to_write_good_tests.md and TODO6.md documenting test quality rules

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Jaehyun Kim <jhkim@precisioninno.com>

spice/test/cpp/TestSpice.cc

@@ -1,4 +1,5 @@
 #include <gtest/gtest.h>
+#include <cmath>
 #include <fstream>
 #include <cstdio>
 #include <unistd.h>
@@ -1794,7 +1795,7 @@ TEST_F(SpiceDesignTest, VertexArrivalForSpice) {
   ASSERT_NE(v, nullptr);
   Arrival arr = sta_->vertexArrival(v, MinMax::max());
   // Arrival should be finite (not INF)
-  (void)arr;
+  EXPECT_FALSE(std::isinf(delayAsFloat(arr)));
 }
 
 // Verify PathExpanded works on timing paths (used in SPICE path writing)