If and only if checks

[kinto-b]

A relatively common formula is

expect_cond(foo, bar)
expect_cond(bar, foo)

This is a bit verbose, particularly so when foo and bar are complicated expressions. We should allow

expect_iff(foo, bar)

or something like

expect_cond(foo, bar, .cond="if")  # foo --> bar 
expect_cond(foo, bar, .cond="iff") # foo <-> bar

[kinto-b]

Simplest implementation, but perhaps not the best due to generating up to two failures

expect_iff <- function(cond1, cond2, ...) {
  cond1 <- rlang::enquo(cond1)
  cond2 <- rlang::enquo(cond2)
  
  testdat::expect_cond(!!cond1, !!cond2, ...)
  testdat::expect_cond(!!cond2, !!cond1, ...)
}

[kinto-b]

Some other logical relationships we might want to support more directly:

# Not both (often used for not both blank)
expect_cond(foo, !bar)
expect_cond(bar, !foo)

# XOR (mostly covered by `expect_exclusive()`)
expect_cond(foo, !bar)
expect_cond(bar, !foo)
expect_cond(TRUE, foo | bar)