[SPARK-20936][CORE] Lack of an important case about the test of resolveURI in UtilsSuite, and add it as needed.

## What changes were proposed in this pull request?
1.  add `assert(resolve(before) === after)` to check before and after in test of resolveURI.
the function `assertResolves(before: String, after: String)` have two params, it means we should check the before value whether equals the after value which we want.
e.g. the after value of Utils.resolveURI("hdfs:///root/spark.jar#app.jar").toString should be "hdfs:///root/spark.jar#app.jar" rather than "hdfs:/root/spark.jar#app.jar". we need `assert(resolve(before) === after)` to make it more safe.
2. identify the cases between resolveURI and resolveURIs.
3. delete duplicate cases and some small fix make this suit more clear.

## How was this patch tested?

unit tests

Author: zuotingbing <[email protected]>

Closes #18158 from zuotingbing/spark-UtilsSuite.

Author: zuotingbing

core/src/test/scala/org/apache/spark/util/UtilsSuite.scala

@@ -461,19 +461,17 @@ class UtilsSuite extends SparkFunSuite with ResetSystemProperties with Logging {
     def assertResolves(before: String, after: String): Unit = {
       // This should test only single paths
       assume(before.split(",").length === 1)
-      // Repeated invocations of resolveURI should yield the same result
       def resolve(uri: String): String = Utils.resolveURI(uri).toString
+      assert(resolve(before) === after)
       assert(resolve(after) === after)
+      // Repeated invocations of resolveURI should yield the same result
       assert(resolve(resolve(after)) === after)
       assert(resolve(resolve(resolve(after))) === after)
-      // Also test resolveURIs with single paths
-      assert(new URI(Utils.resolveURIs(before)) === new URI(after))
-      assert(new URI(Utils.resolveURIs(after)) === new URI(after))
     }
     val rawCwd = System.getProperty("user.dir")
     val cwd = if (Utils.isWindows) s"/$rawCwd".replace("\\", "/") else rawCwd
     assertResolves("hdfs:/root/spark.jar", "hdfs:/root/spark.jar")
-    assertResolves("hdfs:///root/spark.jar#app.jar", "hdfs:/root/spark.jar#app.jar")
+    assertResolves("hdfs:///root/spark.jar#app.jar", "hdfs:///root/spark.jar#app.jar")
     assertResolves("spark.jar", s"file:$cwd/spark.jar")
     assertResolves("spark.jar#app.jar", s"file:$cwd/spark.jar#app.jar")
     assertResolves("path to/file.txt", s"file:$cwd/path%20to/file.txt")
@@ -482,20 +480,19 @@ class UtilsSuite extends SparkFunSuite with ResetSystemProperties with Logging {
       assertResolves("C:\\path to\\file.txt", "file:/C:/path%20to/file.txt")
     }
     assertResolves("file:/C:/path/to/file.txt", "file:/C:/path/to/file.txt")
-    assertResolves("file:///C:/path/to/file.txt", "file:/C:/path/to/file.txt")
+    assertResolves("file:///C:/path/to/file.txt", "file:///C:/path/to/file.txt")
     assertResolves("file:/C:/file.txt#alias.txt", "file:/C:/file.txt#alias.txt")
-    assertResolves("file:foo", s"file:foo")
-    assertResolves("file:foo:baby", s"file:foo:baby")
+    assertResolves("file:foo", "file:foo")
+    assertResolves("file:foo:baby", "file:foo:baby")
   }
 
   test("resolveURIs with multiple paths") {
     def assertResolves(before: String, after: String): Unit = {
       assume(before.split(",").length > 1)
-      assert(Utils.resolveURIs(before) === after)
-      assert(Utils.resolveURIs(after) === after)
-      // Repeated invocations of resolveURIs should yield the same result
       def resolve(uri: String): String = Utils.resolveURIs(uri)
+      assert(resolve(before) === after)
       assert(resolve(after) === after)
+      // Repeated invocations of resolveURIs should yield the same result
       assert(resolve(resolve(after)) === after)
       assert(resolve(resolve(resolve(after))) === after)
     }
@@ -511,6 +508,8 @@ class UtilsSuite extends SparkFunSuite with ResetSystemProperties with Logging {
         s"hdfs:/jar1,file:/jar2,file:$cwd/jar3,file:/C:/pi.py%23py.pi,file:/C:/path%20to/jar4")
     }
     assertResolves(",jar1,jar2", s"file:$cwd/jar1,file:$cwd/jar2")
+    // Also test resolveURIs with single paths
+    assertResolves("hdfs:/root/spark.jar", "hdfs:/root/spark.jar")
   }
 
   test("nonLocalPaths") {