exec/explain: bump test timeout to 5 minutes

We recently added TestMaximumMemoryUsage that is pretty heavy and just
saw the package time out with 1 minute, so let's bump the size to
5 minutes.

Release note: None

Author: yuzefovich

pkg/sql/opt/exec/explain/BUILD.bazel

@@ -52,7 +52,7 @@ go_library(
 
 go_test(
     name = "explain_test",
-    size = "small",
+    size = "medium",
     srcs = [
         "explain_factory_test.go",
         "main_test.go",