[libomptarget] Add a test for OMP_TARGET_OFFLOAD=disabled (#146385)

closes https://github.com/llvm/llvm-project/issues/144786

offload/test/offloading/disable_default_device.c

@@ -0,0 +1,22 @@
+// RUN: %libomptarget-compile-generic
+// RUN:   env OMP_TARGET_OFFLOAD=disabled %libomptarget-run-generic 2>&1 |
+// %fcheck-generic
+
+#include <omp.h>
+#include <stdio.h>
+
+// Sanity checks to make sure that this works and is thread safe.
+int main() {
+  // CHECK: number of devices 0
+  printf("number of devices %d\n", omp_get_num_devices());
+  // CHECK:initial device 0
+  printf("initial device %d\n", omp_get_initial_device());
+  // CHECK:default device 0
+  printf("default device %d\n", omp_get_default_device());
+  // CHECK: PASS
+  if (omp_get_initial_device() == omp_get_default_device()) {
+    printf("PASS\n");
+    return 0;
+  }
+  return 1;
+}