update run_builder, build_sources integration tests for more coverage

spec/build_tests_spec.rb

@@ -647,10 +647,21 @@ EOF
     expect(result.stderr).to eq ""
     expect(lines(result.stdout)).to eq [
       "CC simple.o",
+      "CC two.o",
       "MyProgram simple.exe",
     ]
   end
 
+  it "prints the failed build command for a threaded builder when called via Environment#run_builder without delayed execution" do
+    test_dir("simple")
+    File.open("simple.c", "wb") do |fh|
+      fh.write("FOOBAR")
+    end
+    result = run_test(rsconsfile: "run_builder.rb")
+    expect(result.stderr).to match /Failed to build/
+    expect(result.stdout).to match /Failed command was: gcc/
+  end
+
   context "Directory builder" do
     it "creates the requested directory" do
       test_dir("simple")