Tidy test.data.table() warnings when packages aren't installed

[mattdowle]

These packages are not really required, contrary to the message and warning. Just if they aren't installed, those tests won't run for them, which is fine for users.

> test.data.table()
Running /home/mdowle/build/R-devel/library/data.table/tests/tests.Rraw 
Loading required package: plyr
Loading required package: ggplot2
Loading required package: hexbin
Loading required package: nlme
Loading required package: xts
Loading required package: gdata
Loading required package: GenomicRanges
Loading required package: caret
Loading required package: knitr
Loading required package: plm
Running test id 166     Tests 167-168 not run. If required call library(ggplot2) first.
Running test id 303     Test 304 not run. If required call library(plyr) first.
Running test id 486     Tests 487 and 488 not run. If required call library(reshape) first.
Running test id 840     Test 841 not run. If required call library(xts) first.
Running test id 1372.2     Tests 1372.3+ not run. If required call library(GenomicRanges) first.
Running test id 1383.2     Tests 1383.3 not run. If required call library(reshape) first.
Tests 1383.4 not run. If required call library(caret) first.
Running test id 1394.015     Test 1395 not run. If required call library(knitr) first.
Running test id 1440     Tests 1441-1444 not run. If required install the 'fr_FR.utf8' locale.
All 4149 tests (lastID=1526.2) in inst/tests/tests.Rraw completed ok in 00:02:40 on Mon Jun 22 17:44:42 2015 (endian=little, sizeof(long double)==16)
There were 11 warnings (use warnings() to see them)
> warnings()
Warning messages:
1: In library(package, lib.loc = lib.loc, character.only = TRUE,  ... :
  there is no package called ‘plyr’
2: In library(package, lib.loc = lib.loc, character.only = TRUE,  ... :
  there is no package called ‘ggplot2’
3: In library(package, lib.loc = lib.loc, character.only = TRUE,  ... :
  there is no package called ‘hexbin’
4: In library(package, lib.loc = lib.loc, character.only = TRUE,  ... :
  there is no package called ‘xts’
5: In library(package, lib.loc = lib.loc, character.only = TRUE,  ... :
  there is no package called ‘gdata’
6: In library(package, lib.loc = lib.loc, character.only = TRUE,  ... :
  there is no package called ‘GenomicRanges’
7: In library(package, lib.loc = lib.loc, character.only = TRUE,  ... :
  there is no package called ‘caret’
8: In library(package, lib.loc = lib.loc, character.only = TRUE,  ... :
  there is no package called ‘knitr’
9: In library(package, lib.loc = lib.loc, character.only = TRUE,  ... :
  there is no package called ‘plm’
10: In library(reshape2, pos = "package:base", logical.return = TRUE) :
  there is no package called ‘reshape2’
11: In library(reshape, pos = "package:base", logical.return = TRUE) :
  there is no package called ‘reshape’
>