test: add some comments for test changes

Author: Aaron Blankstein

src/vm/tests/costs.rs

@@ -695,6 +695,8 @@ fn test_tracked_costs(prog: &str, epoch: StacksEpochId) -> ExecutionCost {
 }
 
 #[test]
+// test each individual cost function can be correctly invoked as
+//  Clarity code executes in Epoch 2.00
 fn test_all() {
     let baseline = test_tracked_costs("1", StacksEpochId::Epoch20);
 
@@ -706,6 +708,8 @@ fn test_all() {
 }
 
 #[test]
+// test each individual cost function can be correctly invoked as
+//  Clarity code executes in Epoch 2.05
 fn epoch_205_test_all() {
     let baseline = test_tracked_costs("1", StacksEpochId::Epoch2_05);
 

src/vm/tests/mod.rs

@@ -103,6 +103,9 @@ impl HeadersDB for UnitTestHeaderDB {
         {
             Some(BITCOIN_REGTEST_FIRST_BLOCK_TIMESTAMP as u64)
         } else {
+            // for non-genesis blocks, just pick a u64 value that will increment in most
+            // unit tests as blocks are built (most unit tests construct blocks using
+            // incrementing high order bytes)
             Some(1 + 10 * (id_bhh.as_bytes()[0] as u64))
         }
     }