Check snapshotting reporter when expectations run outside of tests

[hadley]

eg,

Loading required package: DiceKriging
-- ERROR (test-km.R:43:1): (code run outside of `test_that()`) -----------------
Error: no such index at level 1

Backtrace:
    █
 1. └─DiceKriging:::test_that.km(...) test-km.R:43:0
 2.   └─testthat::test_that(...) test-km.R:17:4
 3.     └─testthat:::test_code(desc, code, env = parent.frame(), reporter = reporter)
 4.       └─reporter$start_test(context = reporter$.context, test = test)
 5.         └─testthat:::o_apply(self$reporters, "start_test", context, test)
 6.           └─base::lapply(objects, f)
 7.             └─testthat:::FUN(X[[i]], ...)
 8.               └─x$start_test(...)

[hadley]

Root cause was constructed test call like test_that(c("a", "b"), ...)