Use quasiquotation

[hadley]

To better support parameterised tests. Should include #524

• test_that() needs to use enquo() and test_code() needs to use eval_tidy(). Might be root cause of development version of testthat breaks packrat tests #616. This will also need changes to test_rd() and test_example().

• label() needs to use enexpr() and expr_label() (may need to port multi-line function fix from f7dd41b)

• expect_* needs to enquo() and then immediately evaluate.

[hadley]

Basic idea:

bad_sum <- function(...) 0

val1 <- 1
val2 <- 1
exp <- 3
expect_equal(bad_sum(!!val1, !!val2), !!exp)

# Should generate same failure message as 
expect_equal(bad_sum(1, 1), 3)

#> Error: bad_sum(1, 1) not equal to 3.
#> 1/1 mismatches
#> [1] 10 - 3 == 7 

This is not tidy eval, but quasiquotation + quosures in order to generate better labels.

[hadley]

make_label() is only exported in the dev version, so it's safe to replace.