Set R_TESTS to improve consistency with R CMD check

R CMD check sets this environment variable when running tests, but
tools::testInstalledPackage does not. Covr aims to be as consistent as
possible with R CMD check, so we should set it as well.

Fixes #420

Author: jimhester

NEWS.md

@@ -1,5 +1,7 @@
 # covr (development version)
 
+* `package_coverage()` now sets the environment variable `R_TESTS` to the tests-startup.R file like R CMD check does (#420)
+
 * `codecov()` is now more robust when `coverage` is not the output from `package_coverage()` and `token` is not provided (#456)
 * `as.data.frame()` now returns an 0 row data frame when there are no functions in a package (#427)
 

R/covr.R

@@ -336,7 +336,8 @@ package_coverage <- function(path = ".",
       R_LIBS = libs,
       R_LIBS_USER = libs,
       R_LIBS_SITE = libs,
-      R_COVR = "true"), {
+      R_COVR = "true",
+      R_TESTS = file.path(R.home("share"), "R", "tests-startup.R")), {
 
 
     withCallingHandlers({