ci: increase multi-OS test timeout from 10 minutes to 15 minutes

[nsrip-dd]

What does this PR do?

Increase the timeout for multi-OS unit tests to 15 minutes.

Motivation

The ddtrace/tracer tests take longer than 10 minutes to run on the
GitHub Windows runners. Since 10 minutes is the default Go test timeout,
that means the tests are now failing to complete. Increase the timeout
to 15 minutes to give more headroom until we can figure out why the
tests are taking so long.

Describe how to test/QA your changes

Run the unit tests on Windows with this increased timeout, see if they pass.

Reviewer's Checklist

• If known, an appropriate milestone has been selected; otherwise the Triage milestone is set.
• Changed code has unit tests for its functionality.
• If this interacts with the agent in a new way, a system test has been added.

[pr-commenter]

Benchmarks

Comparing candidate commit 874b8b4 in PR branch nick.ripley/bump-windows-ci-timeout with baseline commit a76b1cb in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 6 cases.

[nsrip-dd]

I've manually dispatched the multi-OS tests on Windows to see if they pass with the increased timeout: https://github.com/DataDog/dd-trace-go/actions/runs/3795352800/jobs/6454387438

We can hold off on merging this until we get a run to pass, in case I need to bump the timeout more.

[nsrip-dd]

The Windows tests succeeded on this branch. ddtrace/tracer took 10 minutes and 30 seconds, so just over the default 10 minute timeout. 15 minutes should be enough for now.