test(diff): add tests for not showing visualization

tests/screenshots/tests-test_diff.lua---Visualization---can-be-visually-disabled

@@ -0,0 +1,23 @@
+--|---------|-----
+01|AAA            
+02|uuu            
+03|BBB            
+04|CcC            
+05|DDD            
+06|FFF            
+07|~              
+08|~              
+09|<e] [+] 1,1 All
+10|               
+
+--|---------|-----
+01|000000000000000
+02|000000000000000
+03|000000000000000
+04|000000000000000
+05|000000000000000
+06|000000000000000
+07|111111111111111
+08|111111111111111
+09|222222222222222
+10|333333333333333

tests/screenshots/tests-test_diff.lua---Visualization---does-not-appear-if-there-is-no-gutter

@@ -0,0 +1,23 @@
+--|---------|-----
+01|AAA            
+02|uuu            
+03|BBB            
+04|CcC            
+05|DDD            
+06|FFF            
+07|~              
+08|~              
+09|<e] [+] 1,1 All
+10|               
+
+--|---------|-----
+01|000000000000000
+02|000000000000000
+03|000000000000000
+04|000000000000000
+05|000000000000000
+06|000000000000000
+07|111111111111111
+08|111111111111111
+09|222222222222222
+10|333333333333333

tests/test_diff.lua

@@ -1957,6 +1957,30 @@ T['Visualization']['works'] = function()
   })
 end
 
+T['Visualization']['does not appear if there is no gutter'] = function()
+  child.o.signcolumn, child.o.number = 'no', false
+  disable()
+  enable()
+
+  set_lines({ 'AAA', 'uuu', 'BBB', 'CcC', 'DDD', 'FFF' })
+  set_ref_text(0, { 'AAA', 'BBB', 'CCC', 'DDD', 'EEE', 'FFF' })
+  -- Shouldn't force neither sign nor number column
+  child.expect_screenshot()
+end
+
+T['Visualization']['can be visually disabled'] = function()
+  child.lua('MiniDiff.config.view.style = "sign"')
+  child.lua('MiniDiff.config.view.signs = { add = "", change = "", delete = "" }')
+  disable()
+  enable()
+
+  set_lines({ 'AAA', 'uuu', 'BBB', 'CcC', 'DDD', 'FFF' })
+  set_ref_text(0, { 'AAA', 'BBB', 'CCC', 'DDD', 'EEE', 'FFF' })
+  -- Shouldn't show sign column, although extmarks are still placed
+  child.expect_screenshot()
+  eq(#get_viz_extmarks(0) > 0, true)
+end
+
 T['Visualization']['works with "add" hunks'] = function()
   -- Every added line should be visualized as part of hunk
   set_lines({ 'aaa', 'uuu', 'vvv', 'bbb', 'www', 'ccc' })