fix warnings in tests

[mbjones]

Warnings from the plot_flower() test:

> test()
Loading flowers
Testing flowers
✔ |  OK F W S | Context
✔ |   2   3   | plot_flower [1.0 s]
────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
test-plot_flower.R:21: warning: flower plot displays
Removed 12 rows containing missing values (position_stack).

test-plot_flower.R:21: warning: flower plot displays
Removed 1 rows containing missing values (position_stack).

test-plot_flower.R:21: warning: flower plot displays
Removed 1 rows containing missing values (geom_errorbar).
────────────────────────────────────────────────────────────────────────────────────────────────────────────────────

══ Results ═════════════════════════════════════════════════════════════════════════════════════════════════════════
Duration: 1.0 s

OK:       2
Failed:   0
Warnings: 3
Skipped:  0
>