Fixed PFD testGetFd_canRead test failed on macOS.

Used `new FileInputStream(FileDescriptor)` instead of `new FileInputStream(String)`.

Avoid depending on Linux file system details, and make full use of Java cross platform feature.

Signed-off-by: ZSmallX <diosmallx@gmail.com>

robolectric/src/test/java/org/robolectric/shadows/ShadowParcelFileDescriptorTest.java

@@ -2,20 +2,20 @@
 
 import static com.google.common.truth.Truth.assertThat;
 import static org.junit.Assert.assertThrows;
-import static org.junit.Assume.assumeThat;
 
 import android.os.ParcelFileDescriptor;
 import androidx.test.core.app.ApplicationProvider;
 import androidx.test.ext.junit.runners.AndroidJUnit4;
 import java.io.File;
+import java.io.FileDescriptor;
 import java.io.FileInputStream;
 import java.io.FileOutputStream;
 import java.io.IOException;
-import org.hamcrest.Matchers;
 import org.junit.After;
 import org.junit.Before;
 import org.junit.Test;
 import org.junit.runner.RunWith;
+import org.robolectric.util.ReflectionHelpers;
 
 @RunWith(AndroidJUnit4.class)
 public class ShadowParcelFileDescriptorTest {
@@ -255,13 +255,15 @@ public void testCreatePipeTwice() throws IOException {
 
   @Test
   public void testGetFd_canRead() throws IOException {
-    assumeThat("Windows is an affront to decency.",
-        File.separator, Matchers.equalTo("/"));
-
     pfd = ParcelFileDescriptor.open(readOnlyFile, ParcelFileDescriptor.MODE_READ_ONLY);
     int fd = pfd.getFd();
     assertThat(fd).isGreaterThan(0);
-    FileInputStream is = new FileInputStream(new File("/proc/self/fd/" + fd));
+
+    final FileDescriptor fD = pfd.getFileDescriptor();
+    assertThat(fD.valid()).isTrue();
+    assertThat(fd).isEqualTo(ReflectionHelpers.getField(fD, "fd"));
+
+    FileInputStream is = new FileInputStream(fD);
     assertThat(is.read()).isEqualTo(READ_ONLY_FILE_CONTENTS);
     is.close();
   }