Don't detach rr workers in SharedArrays tests

By default, I'm having the test suite set up to put any workers spawned
with addprocs into their own rr sessions. This doesn't work with SharedArrays,
because those workers share memory with the main process. Add a new JULIA_RR
environment variable that specifies which rr to use for the test suite and
a flag to the testsuite's addprocs command to determine whether or not to
detach the workers from the current rr session.

stdlib/SharedArrays/test/runtests.jl

@@ -3,7 +3,9 @@
 using Test, Distributed, SharedArrays, Random
 include(joinpath(Sys.BINDIR, "..", "share", "julia", "test", "testenv.jl"))
 
-addprocs_with_testenv(4)
+# These processes explicitly want to share memory, we can't have
+# them in separate rr sessions
+addprocs_with_testenv(4; rr_allowed=false)
 @test nprocs() == 5
 
 @everywhere using Test, SharedArrays

test/testenv.jl

@@ -26,7 +26,16 @@ if !@isdefined(testenv_defined)
         const test_exename = popfirst!(test_exeflags.exec)
     end
 
-    addprocs_with_testenv(X; kwargs...) = addprocs(X; exename=test_exename, exeflags=test_exeflags, kwargs...)
+    if haskey(ENV, "JULIA_RR")
+        const rr_exename = `$(Base.shell_split(ENV["JULIA_RR"]))`
+    else
+        const rr_exename = ``
+    end
+
+    function addprocs_with_testenv(X; rr_allowed=true, kwargs...)
+        exename = rr_allowed ? `$rr_exename $test_exename` : test_exename
+        addprocs(X; exename=exename, exeflags=test_exeflags, kwargs...)
+    end
 
     const curmod = @__MODULE__
     const curmod_name = fullname(curmod)