Detect and use CM files for smlnj (#884)

* Detect and use CM files for smlnj

* Split into two checkers

- one for CM projects
- one for single SML files

* Fix some typos

* Fix error caught by writing tests

We want to actually use `glob` to search in paths upwards from us.
(Previously we were just searching in the current directory every time!)

* Fix errors from former test run

* Write tests for GetCmFile and GetExecutableSmlnj

* Typo in 'smlnj/' fixture filenames

test/test_sml_command.vader

@@ -0,0 +1,47 @@
+Before:
+  runtime ale_linters/sml/sml.vim
+  runtime ale_linters/sml/smlnj.vim
+  call ale#test#SetDirectory('/testplugin/test')
+
+After:
+  call ale#test#RestoreDirectory()
+  call ale#linter#Reset()
+
+# ----- GetCmFile -----
+
+Execute(smlnj finds CM file if it exists):
+  call ale#test#SetFilename('smlnj/cm/foo.sml')
+
+  AssertEqual '/testplugin/test/smlnj/cm/sources.cm', ale#handlers#sml#GetCmFile(bufnr('%'))
+
+Execute(smlnj finds CM file by searching upwards):
+  call ale#test#SetFilename('smlnj/cm/path/to/bar.sml')
+
+  AssertEqual '/testplugin/test/smlnj/cm/sources.cm', ale#handlers#sml#GetCmFile(bufnr('%'))
+
+Execute(smlnj returns '' when no CM file found):
+  call ale#test#SetFilename('smlnj/file/qux.sml')
+
+  AssertEqual '', ale#handlers#sml#GetCmFile(bufnr('%'))
+
+# ----- GetExecutableSmlnjCm & GetExecutableSmlnjFile -----
+
+Execute(CM-project mode enabled when CM file found):
+  call ale#test#SetFilename('smlnj/cm/foo.sml')
+
+  AssertEqual 'sml', ale#handlers#sml#GetExecutableSmlnjCm(bufnr('%'))
+
+Execute(single-file mode disabled when CM file found):
+  call ale#test#SetFilename('smlnj/cm/foo.sml')
+
+  AssertEqual '', ale#handlers#sml#GetExecutableSmlnjFile(bufnr('%'))
+
+Execute(CM-project mode disabled when CM file not found):
+  call ale#test#SetFilename('smlnj/file/qux.sml')
+
+  AssertEqual '', ale#handlers#sml#GetExecutableSmlnjCm(bufnr('%'))
+
+Execute(single-file mode enabled when CM file found):
+  call ale#test#SetFilename('smlnj/file/qux.sml')
+
+  AssertEqual 'sml', ale#handlers#sml#GetExecutableSmlnjFile(bufnr('%'))