test: add missing snapshot for tertiary_contacts() error case

The snapshot for the 'tertiary_contacts errors for unsupported organism'
test added in 5cdd313 was never committed. On CI rcmdcheck runs with
error_on='warning', and the missing snapshot becomes a new-snapshot
warning that fails the R CMD check step.

Author: jayhesselberth

tests/testthat/_snaps/tertiary.md

@@ -0,0 +1,10 @@
+# tertiary_contacts errors for unsupported organism
+
+    Code
+      tertiary_contacts("Mus musculus")
+    Condition
+      Error in `tertiary_contacts()`:
+      ! Organism "Mus musculus" is not supported.
+      i Supported organisms: 'Escherichia coli', 'Saccharomyces cerevisiae', 'Homo sapiens'.
+      i Use `identity_organisms()` to list them.
+