add test that was creating a panic

internal/flow/testdata/import_file/import_file_17.txtar

@@ -0,0 +1,65 @@
+This is a simple test with several levels of nesting.
+This test would often panic when re-evaluation could run concurrently with a config reload.
+Now that it is protected with mutex, this test should always pass.
+
+-- main.river --
+import.file "a_namespace" {
+	filename = "module.river"
+}
+
+a_namespace.a "default" {
+	a_argument = 47
+}
+
+testcomponents.summation "sum" {
+	input = 10
+}
+
+-- module.river --
+declare "a" {
+	import.file "b_namespace" {
+		filename = "nested_module.river"
+	}
+
+	argument "a_argument" {
+		comment = "Where to send collected metrics."
+	}
+
+	b_namespace.b "default" {
+		b_argument = 147
+	}
+
+	export "a_export" {
+		value = argument.a_argument.value + b_namespace.b.default.b_export + 1
+	}
+}
+
+-- nested_module.river --
+declare "b" {
+	import.file "c_namespace" {
+		filename = "other_nested_module.river"
+	}
+
+	argument "b_argument" {
+		optional = false
+	}
+
+	c_namespace.c "default" {
+		c_argument = 101
+	}
+
+	export "b_export" {
+		value = argument.b_argument.value + c_namespace.c.default.c_export + 1
+	}
+}
+
+-- other_nested_module.river --
+declare "c" {
+	argument "c_argument" {
+		optional = false
+	}
+
+	export "c_export" {
+		value = argument.c_argument.value + 1
+	}
+}