test: add integration tests for Test-PSBuildPester (fixes #102)

[nthbotast]

Summary

• add a new tests/Test-PSBuildPester.tests.ps1 integration test file
• cover successful invocation and verify a result object is returned
• cover failing tests and assert Test-PSBuildPester throws (regression Changes $configuration.Run.PassThru to $True #52)
• cover code coverage output path/format and assert a JaCoCo report is generated (regression Addressing Issue #61 - update CodeCoverage Output File Format to fix error #62)

Notes

• fixtures are self-contained by creating temporary test roots per scenario
• local execution is pending here because pwsh is not available in this environment