add TestData.String and use it for logging

[RaduBerinde]

TestData.String() renders the entire testcase and we use it for better error and logging output.

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This change is 

[Copilot]

Copilot reviewed 2 out of 4 changed files in this pull request and generated no comments.

Files not reviewed (2)

• testdata/multiline: Language not supported
• testdata/string: Language not supported

[RaduBerinde]

Also added a t.Helper() so it shows the source which runs the datadriven test.

Since I'm here, would it help to indent the inputs/outputs and diffs? Current output:

    datadriven_test.go:389: 
        testdata/string:9:
        with some args=2
        input-line-1
        input-line-2
        ----
        with some args=2
        input-line-1
        input-line-2
    datadriven_test.go:389: 
        testdata/string:17:
        args that=1 render=(1, 2) differently=(1, 2, 3)
        input-line-1
        input-line-2
        ----
        args that=1 render=(1, 2) differently=(1, 2, 3)
        input-line-1
        input-line-2
    datadriven_test.go:389: 
        testdata/string:25:
        noinput
        ----
        noinput
    datadriven_test.go:389: 
        testdata/string:29:
        fail
        
        expected:
        foo
        
        found:
        fail

[RaduBerinde]

Indented would look like this (looks more readable to me):

    datadriven_test.go:389: 
        testdata/string:9:
          with some args=2
          input-line-1
          input-line-2
          ----
          with some args=2
          input-line-1
          input-line-2
    datadriven_test.go:389: 
        testdata/string:17:
          args that=1 render=(1, 2) differently=(1, 2, 3)
          input-line-1
          input-line-2
          ----
          args that=1 render=(1, 2) differently=(1, 2, 3)
          input-line-1
          input-line-2
    datadriven_test.go:389: 
        testdata/string:25:
          noinput
          ----
          noinput
    datadriven_test.go:389: 
        testdata/string:29:
          fail
        
        expected:
          foo
        
        found:
          fail

[yuzefovich]

I like the proposed indentation.

Reviewed 4 of 4 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @jbowens)

[RaduBerinde]

TFTR! Added indentation.