Fix variability of results in the Second2Gray() snapshot tests.

This sets the measurement date explicitly, as otherwise it would be set to
the current date and cause failures in these tests (see also 83d17ce).

tests/testthat/test_Second2Gray.R

@@ -1,7 +1,8 @@
 data(ExampleData.DeValues, envir = environment())
 
 results <- Second2Gray(ExampleData.DeValues$BT998, c(0.2,0.01))
-dose.rate <- calc_SourceDoseRate(calib.date = "2014-12-19",
+dose.rate <- calc_SourceDoseRate(measurement.date = "2024-10-09",
+                                 calib.date = "2014-12-19",
                                  calib.dose.rate = 0.2, calib.error = 0.01)
 
 test_that("input validation", {