feat: Increase test coverage for pkg/mining

This commit introduces a number of new tests for the `pkg/mining` package,
increasing the overall test coverage from 8.2% to 41.4%.

The following changes were made:

- Added tests for the `XMRigMiner` struct, including its methods for
  installation, starting, stopping, and getting stats.
- Added tests for the `Service` layer, including the API endpoints for
  listing, starting, stopping, and getting stats for miners.
- Added tests for the `Manager`, including starting and stopping multiple
  miners, collecting stats, and getting hashrate history.
- Introduced a `ManagerInterface` to decouple the `Service` layer from the
  concrete `Manager` implementation, facilitating testing with mocks.
- Fixed a failing test on Windows by creating a Windows-compatible
  dummy executable.
- Improved encapsulation in tests by adding and using getter methods for
  private fields.

pkg/mining/xmrig.go

@@ -574,6 +574,18 @@ func (m *XMRigMiner) AddHashratePoint(point HashratePoint) {
 
 // ReduceHashrateHistory aggregates older high-resolution data into 1-minute averages
 // and adds them to the low-resolution history.
+func (m *XMRigMiner) GetHighResHistoryLength() int {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	return len(m.HashrateHistory)
+}
+
+func (m *XMRigMiner) GetLowResHistoryLength() int {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	return len(m.LowResHashrateHistory)
+}
+
 func (m *XMRigMiner) ReduceHashrateHistory(now time.Time) {
 	m.mu.Lock()
 	defer m.mu.Unlock()

pkg/mining/xmrig_test.go

@@ -210,14 +210,12 @@ func TestXMRigMiner_HashrateHistory(t *testing.T) {
 	miner.ReduceHashrateHistory(future)
 
 	// After reduction, high-res history should be smaller
-	miner.mu.Lock()
-	if len(miner.HashrateHistory) >= 10 {
-		t.Errorf("High-res history not reduced, size: %d", len(miner.HashrateHistory))
+	if miner.GetHighResHistoryLength() >= 10 {
+		t.Errorf("High-res history not reduced, size: %d", miner.GetHighResHistoryLength())
 	}
-	if len(miner.LowResHashrateHistory) == 0 {
+	if miner.GetLowResHistoryLength() == 0 {
 		t.Error("Low-res history not populated")
 	}
-	miner.mu.Unlock()
 
 	combinedHistory := miner.GetHashrateHistory()
 	if len(combinedHistory) == 0 {