Merge branch 'jk/enable-test-lint-by-default'

We have two simple and quick tests to catch common mistakes when
writing test scripts, but we did not run them by default when
running tests.

* jk/enable-test-lint-by-default:
  tests: turn on test-lint by default

t/Makefile

@@ -13,6 +13,7 @@ TAR ?= $(TAR)
 RM ?= rm -f
 PROVE ?= prove
 DEFAULT_TEST_TARGET ?= test
+TEST_LINT ?= test-lint-duplicates test-lint-executable
 
 # Shell quote;
 SHELL_PATH_SQ = $(subst ','\'',$(SHELL_PATH))