Adds coverage to two blocks of code in IntegratorBase.

[edrumwri]

Resolves #12329

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This change is 

[edrumwri]

+@sherm1 for feature review please

Reviewable status: LGTM missing from assignee sherm1(platform), needs at least two assigned reviewers (waiting on @sherm1)

[sherm1]

CI failed due to IntegratorBase linting.
Feature pending some clarifications.
+@sammy-tri for platform review per rotation, please

Reviewed 2 of 2 files at r1.
Reviewable status: 3 unresolved discussions, LGTM missing from assignee sammy-tri(platform) (waiting on @edrumwri and @sammy-tri)

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systems/analysis/test/integrator_base_test.cc, line 82 at r1 (raw file):

  // Two values for at_minimum_step_size.
  bool at_minimum_step_size_true = true;
  bool at_minimum_step_size_false = false;

minor: I'm confused about these names, especially when the code below expects at_minimum_step_size_true==false. Please add some comments explaining these names and what they are supposed to do.

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systems/analysis/test/integrator_base_test.cc, line 93 at r1 (raw file):

  std::pair<bool, double> result;
  result = integrator.CalcAdjustedStepSize(zero_error, step_taken,
                                            &at_minimum_step_size_true);

Is this parameter in/out or just out?

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systems/analysis/test/integrator_base_test.cc, line 125 at r1 (raw file):

  // Reset the minimum step size Booleans.
  at_minimum_step_size_true = true;
  at_minimum_step_size_false = false;

minor: you just checked that they are set to these values above

[sammy-tri]

Reviewed 1 of 2 files at r1.
Reviewable status: 3 unresolved discussions, LGTM missing from assignee sammy-tri(platform) (waiting on @edrumwri)

[edrumwri]

Reviewable status: 1 unresolved discussion, LGTM missing from assignee sammy-tri(platform), commits need curation (https://drake.mit.edu/reviewable.html#curated-commits) (waiting on @sammy-tri and @sherm1)

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systems/analysis/test/integrator_base_test.cc, line 93 at r1 (raw file):

Previously, sherm1 (Michael Sherman) wrote…

Is this parameter in/out or just out?

Documentation now points out that these are input and output parameters.

[sammy-tri]

Reviewed 2 of 2 files at r2.
Reviewable status: 1 unresolved discussion, commits need curation (https://drake.mit.edu/reviewable.html#curated-commits) (waiting on @sherm1)

[sherm1]

Reviewed 2 of 2 files at r2.
Reviewable status: commits need curation (https://drake.mit.edu/reviewable.html#curated-commits)