Add tests to improve coverage

Author: LynxJinyangii

RcppTskit/tests/testthat/test_variant_iterator_low_level.R

@@ -57,6 +57,14 @@ test_that("low-level variant iterator validates bounds", {
   ts_xptr <- rtsk_treeseq_load(ts_file)
   seq_len <- rtsk_treeseq_get_sequence_length(ts_xptr)
 
+  expect_error(
+    rtsk_variant_iterator_init(ts_xptr, left = NaN, right = seq_len),
+    "left and right must be finite numbers"
+  )
+  expect_error(
+    rtsk_variant_iterator_init(ts_xptr, left = -1, right = seq_len),
+    "left and right must be >= 0"
+  )
   expect_error(
     rtsk_variant_iterator_init(ts_xptr, left = seq_len + 1, right = seq_len),
     "left and right must be <= sequence length"