release-25.4: testutils/floatcmp: increase CloseMargin to handle distributed execution errors

[blathers-crl]

Backport 1/1 commits from #161907 on behalf of @mw5h.

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Previously, the CloseMargin constant was set to
CloseFraction * CloseFraction (1e-28), which was far too small to handle real-world floating-point rounding errors that occur when comparing results from different query execution paths (distributed vs local). This caused the unoptimized-query-oracle roachtest to flake when aggregate functions like stddev_pop and var_samp produced values differing by ~1e-16 due to different aggregation ordering.

Additionally, the test only used approximate float matching (FloatsMatchApprox) on s390x architecture, while using exact significant-digit matching on other platforms, which didn't leverage the CloseMargin tolerance at all.

This commit makes two changes:

1. Increases CloseMargin from CloseFraction * CloseFraction (1e-28) to CloseFraction / 10 (1e-15), which is appropriate for catching the numerical noise observed in distributed aggregate computations.

2. Changes unsortedMatricesDiffWithFloatComp to always use FloatsMatchApprox for float comparisons on all architectures, not just s390x. This ensures the CloseMargin tolerance is consistently applied.

Fixes #150902

Release note: none

Epic: None

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Release justification:

[blathers-crl]

Thanks for opening a backport.

Before merging, please confirm that it falls into one of the following categories (select one):

• Non-production code changes. Includes test-only changes, build system changes, etc.
• Fixes for serious issues. Defined in the policy as correctness, stability, or security issues, data corruption/loss, significant performance regressions, breaking working and widely used functionality, or an inability to detect and debug production issues.
• Other approved changes. These changes must be gated behind a disabled-by-default feature flag unless there is a strong justification not to.

Add a brief release justification to the PR description explaining your selection.

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[trunk-io]

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[blathers-crl]

✅ PR #162223 is compliant with backport policy

Confidence: high
Backward compatible: true
Explanation: The pull request involves modifications to files that are exclusively within the scope of test utilities (testutils) and testing-related changes in roachtest/tests. According to the CockroachDB backport policy, changes affecting non-production code, such as testing infrastructure, are exempt from the requirements to fix a critical bug or need to be gated behind a feature flag. Since the affected files (floatcmp.go, floatcmp_test.go, and query_comparison_util.go) fall under test utilities, which are considered non-production, the pull request is compliant without needing further justification via 'Release justification' or criteria for critical bugs.

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[cockroach-teamcity]

This change is