lntest: make mining block limit configurable

Nudging test authors towards not mining too many blocks makes sense,
especially in lnd where we have a lot of integration tests.
But the lntest package is also used in other projects where this
restriction might lead to large refactors.
To be able to stage those refactors we also want to allow this limit to
be configurable if lntest is used as a library.

lntest/harness.go

@@ -59,6 +59,13 @@ const (
 	thawHeightDelta = finalCltvDelta * 2   // 36.
 )
 
+var (
+	// MaxBlocksMinedPerTest is the maximum number of blocks that we allow
+	// a test to mine. This is an exported global variable so it can be
+	// overwritten by other projects that don't have the same constraints.
+	MaxBlocksMinedPerTest = 50
+)
+
 // TestCase defines a test case that's been used in the integration test.
 type TestCase struct {
 	// Name specifies the test name.
@@ -396,10 +403,14 @@ func (h *HarnessTest) checkAndLimitBlocksMined(startHeight int32) {
 		"5. use `CreateSimpleNetwork` for efficient channel creation.\n"
 	h.Log(desc)
 
-	// We enforce that the test should not mine more than 50 blocks, which
-	// is more than enough to test a multi hop force close scenario.
-	require.LessOrEqual(h, int(blocksMined), 50, "cannot mine more than "+
-		"50 blocks in one test")
+	// We enforce that the test should not mine more than
+	// MaxBlocksMinedPerTest (50 by default) blocks, which is more than
+	// enough to test a multi hop force close scenario.
+	require.LessOrEqualf(
+		h, int(blocksMined), MaxBlocksMinedPerTest,
+		"cannot mine more than %d blocks in one test",
+		MaxBlocksMinedPerTest,
+	)
 }
 
 // shutdownNodesNoAssert will shutdown all running nodes without assertions.