sfp by default attempts to trigger all the apex tests in a package in asynchronous mode during validation. This is to ensure that you have a highly performant apex test classes which provide you fast feedback.

If you have inherited an existing code base or your unit tests have lot of DML statemements, you will encounter test failuers when a tests in package is triggered asynchronously. You can instruct sfp to trigger the tests of the package in a synchronous mode by setting 'testSynchnronous as true

// Demonstrating how to do use testSynchronous
{
  "packageDirectories": [
    {
      "path": "core-crm",
      "package": "core-crm",
      "versionDescription": "Package containing core schema and classes",
      "versionNumber": "4.7.0.NEXT",
      
    },
     ...
   ]
}

sfp during validation checks the apex test coverage of a package depending on the package type. This is beneficial so that you dont run into any coverage issues while deploying into higher environments or building an unlocked package.

However, there could be situations where the test coverage calculation is flaky , sfp provides you with an option to turn the coverage validation off.

// Demonstrating how to do use skipCoverageValidation
{
  "packageDirectories": [
    {
      "path": "core-crm",
      "package": "core-crm",
      "versionDescription": "Package containing core schema and classes",
      "versionNumber": "4.7.0.NEXT",
      
    },
     ...
   ]
}

One can utilize this attribute on a package definition is sfdx-project.json to skipTesting while a change is validated. Use this option with caution, as the package when it gets deployed (depending on the package behaviour) might trigger testing while being deployed to production

// Demonstrating how to do use skipTesting
{
  "packageDirectories": [
    {
      "path": "core-crm",
      "package": "core-crm",
      "versionDescription": "Package containing core schema and classes",
      "versionNumber": "4.7.0.NEXT",
      
    },
     ...
   ]
}