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

[sun-rui]

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)'.

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Merged build triggered.

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[SparkQA]

Test build #36238 has started for PR 7152 at commit 8fe9f0c.

[SparkQA]

Test build #36238 has finished for PR 7152 at commit 8fe9f0c.

• This patch fails SparkR unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

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Merged build finished. Test FAILed.

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Merged build triggered.

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Merged build started.

[SparkQA]

Test build #36251 has started for PR 7152 at commit 8ad2440.

[SparkQA]

Test build #36251 has finished for PR 7152 at commit 8ad2440.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

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Merged build finished. Test PASSed.

[shivaram]

LGTM. Thanks @sun-rui for this change !