crypto/x509: TestBoringAllowCert failures [1.19 backport]

[gopherbot]

@bcmills requested issue #56798 to be considered for backport to the next 1.19 minor release.

@gopherbot, please backport to Go 1.19. This test is still flaky on the release branch.

[cagedmantis]

This is approved as it is a test only change and it's something that has impacted the release in the past.

[gopherbot]

Change https://go.dev/cl/461615 mentions this issue: [release-branch.go1.19] crypto/x509: reduce boring test key size

[gopherbot]

Closed by merging f38c1eb to release-branch.go1.19.