fix: Restore accidentally deleted test files

**Test Files Restored:**
- tests/database_services_tests.rs (595 lines)
- tests/microvm_integration_tests.rs (475 lines)
- tests/snapshot_microvm_security_tests.rs (584 lines)
- tests/test_100_microvms.rs (287 lines)

**Reason:**
These files were mistakenly deleted in commit 5b7e23e, which was
supposed to only contain formatting and documentation changes. The
commit message did not accurately reflect the deletion of these test
files, which was an error.

**Impact:**
- Critical microVM, database, and integration tests were removed
- Test coverage dropped significantly
- No functional code was lost, only test infrastructure

**Resolution:**
Restored all files from commit 5b7e23e^ (parent of the deletion commit)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: 0xrinegade