[SYCL] add overlooked default context test . (#12728)

despite having a unit test for default context, realized there is not
one to affirm the new default configuration.

sycl/test-e2e/Config/default-context.cpp

@@ -0,0 +1,22 @@
+// RUN: %{build} -o %t.out
+// RUN: %{run} %t.out
+// RUN: env SYCL_ENABLE_DEFAULT_CONTEXTS=1 %t.out
+// RUN: env SYCL_ENABLE_DEFAULT_CONTEXTS=0 %t.out 1
+
+#include <sycl/sycl.hpp>
+
+// when not using the environment variable, we use the "default context" on both
+// Lin and Win.  This test asserts it defaults correctly, and that the
+// environment variable is working as expected.
+
+// no args: YES default context.
+// any arg: NO default context.
+int main(int argc, char *argv[]) {
+  sycl::queue q1;
+  sycl::queue q2;
+
+  if (argc <= 1)
+    assert(q1.get_context() == q2.get_context());
+  else
+    assert(q1.get_context() != q2.get_context());
+}