Refined RigidBodySystem unit test

[liangfok]

Mostly adding more checks and fixing grammar and spelling issues in the comments.

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This change is

[liangfok]

This PR passed https://drake-jenkins.csail.mit.edu/job/experimental/2116/ and is thus ready for review.
May I suggest:
+@sherm1 for feature review since he reviewed the previous version of this unit test.
+@ggould-tri for platform review since it's Friday.

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Review status: 0 of 1 files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[ggould-tri]

Big readability improvements; thanks!

I have added one item to consider below. However because it is author's discretion I am applying platform anyway and you can decide if you want to bother.

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Reviewed 1 of 1 files at r1.
Review status: all files reviewed at latest revision, 1 unresolved discussion.

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drake/systems/plants/test/rigid_body_system/rigid_body_system_test.cc, line 125 [r1] (raw file):

  EXPECT_NE(tree->findLink("link_3", "model_2", 1), nullptr);
  EXPECT_NE(tree->findLink("link_3", "model_1", 2), nullptr);
  EXPECT_NE(tree->findLink("link_3", "model_2", 3), nullptr);

Consider using a for loop here for conciseness (for (auto link : {"link1", ...}) { for (auto model : {"model_1", ...}) ...)

Author's discretion.

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Comments from Reviewable

[liangfok]

Review status: all files reviewed at latest revision, 1 unresolved discussion.

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drake/systems/plants/test/rigid_body_system/rigid_body_system_test.cc, line 125 [r1] (raw file):

Previously, ggould-tri wrote…

Consider using a for loop here for conciseness (for (auto link : {"link1", ...}) { for (auto model : {"model_1", ...}) ...)

Author's discretion.

Unfortunately, using a `for` loop is not trivial because there are certain combinations that do not exist. For example, there's no such thing as <"link_1", "model_1", 1> or <"link_1", "model_2", 0>. Because of this, I'm going to merge as is.

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Comments from Reviewable

[sherm1]

Feature review: ; merging.

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Review status: all files reviewed at latest revision, all discussions resolved, all commit checks successful.

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Comments from Reviewable