Set R_TESTS environment

Hopefully fixes error when building on windows

DESCRIPTION

 Package: splatter
 Type: Package
 Title: Simple Simulation of Single-cell RNA Sequencing Data
-Version: 0.99.4
-Date: 2016-12-23
+Version: 0.99.5
+Date: 2016-12-28
 Author: Luke Zappia
 Authors@R:
     c(person("Luke", "Zappia", role = c("aut", "cre"),

NEWS.md

+# 0.99.5
+
+* Set R_TESTS environment
+
 # 0.99.4
 
 * Version bump to start build

tests/testthat.R

+# setting R_TESTS to empty string because of
+# https://github.com/hadley/testthat/issues/144
+# revert this when that issue in R is fixed.
+Sys.setenv("R_TESTS" = "")
+
 library(testthat)
 library(splatter)