Slang linter for Verilog. (#4713)

test/linter/test_slang.vader

@@ -0,0 +1,14 @@
+Before:
+  call ale#assert#SetUpLinterTest('verilog', 'slang')
+
+After:
+  call ale#assert#TearDownLinterTest()
+
+Execute(The default slang command should be correct):
+  AssertLinter 'slang', 'slang -Weverything -I%s:h  %t'
+
+Execute(slang options should be configurable):
+  " Additional args for the linter
+  let g:ale_verilog_slang_options = '--define-macro DWIDTH=12'
+
+  AssertLinter 'slang', 'slang -Weverything -I%s:h --define-macro DWIDTH=12 %t'