testing: add a new output writer and use it in t.Log

[suntala]

This is the first of two planned CLs. The first one introduces a new
output writer and uses it in a refactored implementation of t.Log.
This new writer maintains the formatting provided by t.Log while making
call site information optional. Additionally, it provides buffering of
input. The second CL would export a new t.Output method, refine its
implementation and add testing.

Co-authored-by: Aleks Fazlieva [email protected]

For #59928.

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Patch Set 2: -Commit-Queue

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Patch Set 2:

This CL has failed the run. Reason:

Tryjob golang/try/gotip-linux-arm64-boringcrypto has failed with summary (view all results):

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• go on master (change 646956)

Build or test failure, click here for results.

To reproduce, try gomote repro 8723741626891534561.

Additional links for debugging:

• Output for go tool dist test -json [shard 2]

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Patch Set 2:

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Patch Set 2:

This CL has failed the run. Reason:

Tryjob golang/try/gotip-linux-arm64 has failed with summary (view all results):

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• go on master (change 646956)

Build or test failure, click here for results.

To reproduce, try gomote repro 8723739851759057057.

Additional links for debugging:

• Output for go tool dist test -json [shard 1]

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