[SPARK-17412][DOC] All test should not be run by `root` or any admin user

## What changes were proposed in this pull request?

`FsHistoryProviderSuite` fails if `root` user runs it. The test case **SPARK-3697: ignore directories that cannot be read** depends on `setReadable(false, false)` to make test data files and expects the number of accessible files is 1. But, `root` can access all files, so it returns 2.

This PR adds the assumption explicitly on doc. `building-spark.md`.

## How was this patch tested?

This is a documentation change.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #15291 from dongjoon-hyun/SPARK-17412.

docs/building-spark.md

@@ -215,6 +215,7 @@ For help in setting up IntelliJ IDEA or Eclipse for Spark development, and troub
 # Running Tests
 
 Tests are run by default via the [ScalaTest Maven plugin](http://www.scalatest.org/user_guide/using_the_scalatest_maven_plugin).
+Note that tests should not be run as root or an admin user.
 
 Some of the tests require Spark to be packaged first, so always run `mvn package` with `-DskipTests` the first time.  The following is an example of a correct (build, test) sequence: