Write unit tests for `set_alignment()`

[matiasandina]

Here are the unit tests with expected results Ideally this should be incorporated to the examples and tests

library(tidyverse)
library(fed3)
df <- tibble(
  group_col = c(1, 1, 1, 2, 2, 3, 3, 3),
  datetime_col = c(
    as.POSIXct("2023-01-01 12:00:00"),
    as.POSIXct("2023-01-01 12:30:00"),
    as.POSIXct("2023-01-03 10:00:01"),
    as.POSIXct("2023-01-01 13:15:00"),
    as.POSIXct("2023-01-01 13:45:00"),
    as.POSIXct("2023-01-20 10:00:00"),
    as.POSIXct("2023-01-20 11:00:00"),
    as.POSIXct("2023-01-31 16:00:00")
  )
)

df %>% group_by(group_col) %>%
  set_alignment(datetime_col, "midnight")

df %>% group_by(group_col) %>%
   set_alignment(datetime_col, "time")

# One issue here is the change of dttm to drtn, maybe we can consider behaving differently
df %>% group_by(group_col) %>%
  set_alignment(datetime_col, "elapsed")