Declare dependency on bcmath extension

[fredden]

Description

While running the test suite for this repository within a limited environment, several tests were failing without the bcmath PHP extension. This change skips these tests when the bcmath extension is not loaded. Elsewhere, we use this same syntax to skip tests which depend on the bcmath extension.

Suggested changelog entry

None required

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
  • This change is only breaking for integrators, not for external standards or end-users.
• Documentation improvement

PR checklist

• I have checked there is no other PR open for the same change.
• I have read the Contribution Guidelines.
• I grant the project the right to include and distribute the code under the BSD-3-Clause license (and I have the right to grant these rights).
• I have added tests to cover my changes.
• I have verified that the code complies with the projects coding standards.
• [Required for new sniffs] I have added XML documentation for the sniff.

[jrfnl]

@fredden Thanks for this PR. LGTM, the only thing that needs changing, is that this change should be pulled to the 3.x branch as these tests already exist in 3.x.

[jrfnl]

Thanks @fredden ! Will merge this when I'm back in the office.