[SPARK-7714] [SPARKR] SparkR tests should use more specific expectations than expect_true

1. Update the pattern 'expect_true(a == b)' to 'expect_equal(a, b)'.
2. Update the pattern 'expect_true(inherits(a, b))' to 'expect_is(a, b)'.
3. Update the pattern 'expect_true(identical(a, b))' to 'expect_identical(a, b)'.

Author: Sun Rui <rui.sun@intel.com>

Closes #7152 from sun-rui/SPARK-7714 and squashes the following commits:

8ad2440 [Sun Rui] Fix test case errors.
8fe9f0c [Sun Rui] Update the pattern 'expect_true(identical(a, b))' to 'expect_identical(a, b)'.
f1b8005 [Sun Rui] Update the pattern 'expect_true(inherits(a, b))' to 'expect_is(a, b)'.
f631e94 [Sun Rui] Update the pattern 'expect_true(a == b)' to 'expect_equal(a, b)'.