froll, main, mergelist tests: limit to 2 threads (#7465)

Rely on setDTthreads() to read $OMP_THREAD_LIMIT when the package is loaded.

Author: aitap

tests/froll.R

@@ -1,3 +1,4 @@
+Sys.setenv(OMP_THREAD_LIMIT = Sys.getenv("OMP_THREAD_LIMIT", "2"))
 require(data.table)
 test.data.table(script="froll.Rraw")
 test.data.table(script="frollBatch.Rraw", optional=TRUE)

tests/main.R

@@ -1,3 +1,4 @@
+Sys.setenv(OMP_THREAD_LIMIT = Sys.getenv("OMP_THREAD_LIMIT", "2"))
 require(data.table)
 
 test.data.table()  # runs the main test suite of 5,000+ tests in /inst/tests/tests.Rraw

tests/mergelist.R

@@ -1,2 +1,3 @@
+Sys.setenv(OMP_THREAD_LIMIT = Sys.getenv("OMP_THREAD_LIMIT", "2"))
 require(data.table)
 test.data.table(script="mergelist.Rraw")