Pester 5 and redesigned test structure

[nvarscar]

• All syntax has been updated to use Pester 5
• Introduced fixtures that are shared between tests to reduce declarations in each test
• Functional tests now support every DBMS, which enables a much more efficient code reuse
  • Each DBMS can now be targeted by an environment variable DBOPS_TEST_DB_TYPE or by providing parameters to the manual.pester.ps1 file
• Linux testing is now handled by GitHub, Appveyor still exists to support library build and PS5 environment tests