[SPARK-20877][SPARKR] refactor tests to basic tests only for CRAN

## What changes were proposed in this pull request?

Move all existing tests to non-installed directory so that it will never run by installing SparkR package

For a follow-up PR:
- remove all skip_on_cran() calls in tests
- clean up test timer
- improve or change basic tests that do run on CRAN (if anyone has suggestion)

It looks like `R CMD build pkg` will still put pkg\tests (ie. the full tests) into the source package but `R CMD INSTALL` on such source package does not install these tests (and so `R CMD check` does not run them)

## How was this patch tested?

- [x] unit tests, Jenkins
- [x] AppVeyor
- [x] make a source package, install it, `R CMD check` it - verify the full tests are not installed or run

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #18264 from felixcheung/rtestset.

R/pkg/tests/run-all.R

@@ -43,3 +43,11 @@ if (identical(Sys.getenv("NOT_CRAN"), "true")) {
 }
 
 test_package("SparkR")
+
+if (identical(Sys.getenv("NOT_CRAN"), "true")) {
+  # for testthat 1.0.2 later, change reporter from "summary" to default_reporter()
+  testthat:::run_tests("SparkR",
+                       file.path(sparkRDir, "pkg", "tests", "fulltests"),
+                       NULL,
+                       "summary")
+}