ci: replace test_file Make target with generated test_xxx per module

Details:
  - Was: FILE=tests/test_ai.lua make test_file
  - Now: make test_ai

Author: echasnovski

CONTRIBUTING.md

@@ -99,9 +99,9 @@ If your contribution updates code, please make sure that it doesn't break existi
 
 - From command line:
     - Execute `make test` to run all tests (with `nvim` as executable).
-    - Execute `FILE=tests/test_xxx.lua make test_file` to run tests only from file `tests/test_xxx.lua` (with `nvim` as executable).
+    - Execute `make test_xxx` to run tests only from file `tests/test_xxx.lua` (with `nvim` as executable). For example, `make test_ai`.
     - If you have multiple Neovim executables (say, `nvim_07`, `nvim_08`, `nvim_09`, `nvim_010`), you can use `NVIM_EXEC` variable to tests against multiple versions like this:
-      `NVIM_EXEC="nvim_07 nvim_08 nvim_09 nvim_010" make test` or `NVIM_EXEC="nvim_07 nvim_08 nvim_09 nvim_010" FILE=tests/test_xxx.lua make test_file`.
+      `NVIM_EXEC="nvim_07 nvim_08 nvim_09 nvim_010" make test` or `NVIM_EXEC="nvim_07 nvim_08 nvim_09 nvim_010" make test_xxx`.
 - Inside Neovim instance execute `:lua require('mini.test').setup(); MiniTest.run()` to run all tests or `:lua require('mini.test').setup(); MiniTest.run_file()` to run tests only from current buffer.
 
 This plugin uses 'mini.test' to manage its tests. For a more hands-on introduction, see [TESTING.md](TESTING.md).

Makefile

@@ -3,6 +3,7 @@ NVIM_EXEC ?= nvim
 
 all: test documentation
 
+# Use `make test` to run tests for all modules
 test:
 	for nvim_exec in $(NVIM_EXEC); do \
 		printf "\n======\n\n" ; \
@@ -12,13 +13,16 @@ test:
 			-c "lua MiniTest.run({ execute = { reporter = MiniTest.gen_reporter.stdout({ group_depth = $(GROUP_DEPTH) }) } })" ; \
 	done
 
-test_file:
+# Use `make test_xxx` to run tests for module 'mini.xxx'
+TEST_MODULES = $(basename $(notdir $(wildcard tests/test_*.lua)))
+
+$(TEST_MODULES):
 	for nvim_exec in $(NVIM_EXEC); do \
 		printf "\n======\n\n" ; \
 		$$nvim_exec --version | head -n 1 && echo '' ; \
 		$$nvim_exec --headless --noplugin -u ./scripts/minimal_init.lua \
 			-c "lua require('mini.test').setup()" \
-			-c "lua MiniTest.run_file('$(FILE)', { execute = { reporter = MiniTest.gen_reporter.stdout({ group_depth = $(GROUP_DEPTH) }) } })" ; \
+			-c "lua MiniTest.run_file('tests/$@.lua', { execute = { reporter = MiniTest.gen_reporter.stdout({ group_depth = $(GROUP_DEPTH) }) } })" ; \
 	done
 
 documentation: