[tools] Port tests to use runfiles properly

[jwnimmer-tri]

In Bazel 8, we can't just assume runfiles are in the cwd with well-known names.

Towards #22182.

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This change is 

[jwnimmer-tri]

+@xuchenhan-tri for feature review, please.

[xuchenhan-tri]

Reviewed 11 of 11 files at r1, all commit messages.
Reviewable status: 1 unresolved discussion, needs at least two assigned reviewers

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tools/lint/buildifier.py line 24 at r1 (raw file):

    """Returns a list starting with the buildifier executable, followed by any
    required default arguments."""
    manifest = runfiles.Create()

BTW, is runfiles.Create() a new functionality from Bazel? This looks a lot cleaning than the find_data script. Wonder why we don't use it before.

[jwnimmer-tri]

+@sammy-tri for platform review per schedule, please.

Reviewable status: LGTM missing from assignee sammy-tri(platform)

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tools/lint/buildifier.py line 24 at r1 (raw file):

Previously, xuchenhan-tri wrote…

BTW, is runfiles.Create() a new functionality from Bazel? This looks a lot cleaning than the find_data script. Wonder why we don't use it before.

I suppose it is "new" relative to when find_data was originally written in 2017, but it has been around for 5 years (#10659) or probably even earlier. This buildifier wrapper some of the oldest code in Drake, and just never got revisited after we learned about the Rlocation function offered by Bazel.