Fix intermittency in the sys.fs.fusefs.symlink.main test

This change is identical to 86885b1 but for symlink instead of
mknod.  The kernel sends a FUSE_FORGET asynchronously with the final
syscall.  The lack of an expectation caused this test to occasionally
fail.

Also, remove a sleep that accidentally snuck into a different test.

MFC after:	2 weeks
MFC with:	86885b1
Sponsored by:	Axcient

tests/sys/fs/fusefs/mkdir.cc

@@ -241,7 +241,6 @@ TEST_F(Mkdir, parent_inode)
 
 	ASSERT_EQ(-1, mkdir(FULLPATH, mode));
 	ASSERT_EQ(EIO, errno);
-	usleep(100000);
 }
 
 TEST_F(Mkdir_7_8, ok)

tests/sys/fs/fusefs/symlink.cc

@@ -29,6 +29,7 @@
  */
 
 extern "C" {
+#include <semaphore.h>
 #include <unistd.h>
 }
 
@@ -174,15 +175,22 @@ TEST_F(Symlink, parent_ino)
 	const char PPATH[] = "parent";
 	const char RELPATH[] = "src";
 	const char dst[] = "dst";
+	sem_t sem;
 	const uint64_t ino = 42;
 
+	ASSERT_EQ(0, sem_init(&sem, 0, 0)) << strerror(errno);
+
 	expect_lookup(PPATH, ino, S_IFDIR | 0755, 0, 1);
 	EXPECT_LOOKUP(ino, RELPATH)
 	.WillOnce(Invoke(ReturnErrno(ENOENT)));
 	expect_symlink(ino, dst, RELPATH);
+	expect_forget(ino, 1, &sem);
 
 	EXPECT_EQ(-1, symlink(dst, FULLPATH));
 	EXPECT_EQ(EIO, errno);
+
+	sem_wait(&sem);
+	sem_destroy(&sem);
 }
 
 TEST_F(Symlink_7_8, ok)