test(tls): add comprehensive tests for TLS package

[evgeniy-scherbina]

Summary

This PR adds comprehensive test coverage for the TLS certificate manager package to resolve issue #52.

Tests Added

• TestNewCertificateManager - Validates certificate manager initialization and CA generation
• TestSetupTLSAndWriteCACert - Tests TLS configuration setup and CA cert file writing
• TestLoadExistingCA - Verifies loading of existing CA certificates from disk
• TestGetCertificate - Tests certificate generation for hostnames and IP addresses
• TestCertificateCache - Validates caching mechanism for generated certificates
• TestGenerateServerCertificateWithIP - Tests IP address handling in certificates
• TestGetCACertPEM - Tests PEM encoding of CA certificates
• TestConcurrentCertificateGeneration - Validates thread-safe concurrent certificate requests

Test Coverage

All tests validate:

• ✅ Certificate properties and validity periods
• ✅ CA signing and verification chains
• ✅ Thread-safe cache operations
• ✅ IP address and hostname handling
• ✅ PEM encoding/decoding
• ✅ File operations for CA persistence

Test Results

=== RUN   TestNewCertificateManager
--- PASS: TestNewCertificateManager (0.05s)
=== RUN   TestSetupTLSAndWriteCACert
--- PASS: TestSetupTLSAndWriteCACert (0.22s)
=== RUN   TestLoadExistingCA
--- PASS: TestLoadExistingCA (0.14s)
=== RUN   TestGetCertificate
--- PASS: TestGetCertificate (0.66s)
=== RUN   TestCertificateCache
--- PASS: TestCertificateCache (0.38s)
=== RUN   TestGenerateServerCertificateWithIP
--- PASS: TestGenerateServerCertificateWithIP (0.17s)
=== RUN   TestGetCACertPEM
--- PASS: TestGetCACertPEM (0.03s)
=== RUN   TestConcurrentCertificateGeneration
--- PASS: TestConcurrentCertificateGeneration (0.87s)
PASS
ok  	github.com/coder/boundary/tls	2.537s

Closes #52

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