Prepare test setup for CI tests triggered from upstream (#79)

Co-authored-by: Hendrik Ranocha <[email protected]>

test/runtests.jl

@@ -1,7 +1,18 @@
 using Test
 
-@testset "Trixi2Vtk" begin
-  include("test_2d.jl")
-  include("test_3d.jl")
-  include("test_manual.jl")
+# We use the `TRIXI_TEST` environment variable to determine the subset of tests to execute.
+const TRIXI_TEST = get(ENV, "TRIXI_TEST", "all")
+
+@time @testset "Trixi2Vtk" begin
+  @time if TRIXI_TEST == "all" || TRIXI_TEST == "2d" || TRIXI_TEST == "upstream"
+    include("test_2d.jl")
+  end
+
+  @time if TRIXI_TEST == "all" || TRIXI_TEST == "3d" || TRIXI_TEST == "upstream"
+    include("test_3d.jl")
+  end
+
+  @time if TRIXI_TEST == "all" || TRIXI_TEST == "manual"
+    include("test_manual.jl")
+  end
 end