[SPARK-25167][SPARKR][TEST][MINOR] Minor fixes for R sql tests (timestamp comparison)

## What changes were proposed in this pull request?
The "date function on DataFrame" test fails consistently on my laptop. In this PR
i am fixing it by changing the way we compare the two timestamp values. With this change i am able to run the tests clean.

## How was this patch tested?
Fixed the failing test.

Author: Dilip Biswal <[email protected]>

Closes apache#22274 from dilipbiswal/r-sql-test-fix2.

Author: dilipbiswal

R/pkg/tests/fulltests/test_sparkSQL.R

@@ -1870,9 +1870,9 @@ test_that("date functions on a DataFrame", {
   expect_equal(collect(select(df2, minute(df2$b)))[, 1], c(34, 24))
   expect_equal(collect(select(df2, second(df2$b)))[, 1], c(0, 34))
   expect_equal(collect(select(df2, from_utc_timestamp(df2$b, "JST")))[, 1],
-               c(as.POSIXlt("2012-12-13 21:34:00 UTC"), as.POSIXlt("2014-12-15 10:24:34 UTC")))
+               c(as.POSIXct("2012-12-13 21:34:00 UTC"), as.POSIXct("2014-12-15 10:24:34 UTC")))
   expect_equal(collect(select(df2, to_utc_timestamp(df2$b, "JST")))[, 1],
-               c(as.POSIXlt("2012-12-13 03:34:00 UTC"), as.POSIXlt("2014-12-14 16:24:34 UTC")))
+               c(as.POSIXct("2012-12-13 03:34:00 UTC"), as.POSIXct("2014-12-14 16:24:34 UTC")))
   expect_gt(collect(select(df2, unix_timestamp()))[1, 1], 0)
   expect_gt(collect(select(df2, unix_timestamp(df2$b)))[1, 1], 0)
   expect_gt(collect(select(df2, unix_timestamp(lit("2015-01-01"), "yyyy-MM-dd")))[1, 1], 0)
@@ -3652,7 +3652,8 @@ test_that("catalog APIs, currentDatabase, setCurrentDatabase, listDatabases", {
   expect_equal(currentDatabase(), "default")
   expect_error(setCurrentDatabase("default"), NA)
   expect_error(setCurrentDatabase("zxwtyswklpf"),
-        "Error in setCurrentDatabase : analysis error - Database 'zxwtyswklpf' does not exist")
+               paste0("Error in setCurrentDatabase : analysis error - Database ",
+               "'zxwtyswklpf' does not exist"))
   dbs <- collect(listDatabases())
   expect_equal(names(dbs), c("name", "description", "locationUri"))
   expect_equal(which(dbs[, 1] == "default"), 1)