test: reorg #15698

Problem:
Subdirectories like "visual", "insert", "normal" encourage people to
separate *related* tests for no good reason.  Typically the _mode_ is
not the relevant topic of a test (and when it is, _then_ create
an appropriate describe() or it()).

Solution:
- Delete the various `test/functional/<mode>/` subdirectories, move
  their tests to more meaningful topics.
- Rename `…/normal/` to `…/editor/`.
  - Move or merge `…/visual/*` and `…/insert/*` tests into here where
    appropriate.
- Rename `…/eval/` to `…/vimscript/`.
  - Move `…/viml/*` into here also.

* test(reorg): insert/* => editor/mode_insert_spec.lua
* test(reorg): cmdline/* => editor/mode_cmdline_spec.lua
* test(reorg): eval core tests => eval_spec.lua

test/functional/vimscript/environ_spec.lua

@@ -0,0 +1,80 @@
+local helpers = require('test.functional.helpers')(after_each)
+local clear = helpers.clear
+local eq = helpers.eq
+local environ = helpers.funcs.environ
+local exists = helpers.funcs.exists
+local system = helpers.funcs.system
+local nvim_prog = helpers.nvim_prog
+local command = helpers.command
+local eval = helpers.eval
+local setenv = helpers.funcs.setenv
+
+describe('environment variables', function()
+  it('environ() handles empty env variable', function()
+    clear({env={EMPTY_VAR=""}})
+    eq("", environ()['EMPTY_VAR'])
+    eq(nil, environ()['DOES_NOT_EXIST'])
+  end)
+
+  it('exists() handles empty env variable', function()
+    clear({env={EMPTY_VAR=""}})
+    eq(1, exists('$EMPTY_VAR'))
+    eq(0, exists('$DOES_NOT_EXIST'))
+  end)
+end)
+
+describe('empty $HOME', function()
+  local original_home = os.getenv('HOME')
+
+  -- recover $HOME after each test
+  after_each(function()
+    if original_home ~= nil then
+      setenv('HOME', original_home)
+    end
+    os.remove('test_empty_home')
+    os.remove('./~')
+  end)
+
+  local function tilde_in_cwd()
+    -- get files in cwd
+    command("let test_empty_home_cwd_files = split(globpath('.', '*'), '\n')")
+    -- get the index of the file named '~'
+    command('let test_empty_home_tilde_index = index(test_empty_home_cwd_files, "./~")')
+    return eval('test_empty_home_tilde_index') ~= -1
+  end
+
+  local function write_and_test_tilde()
+    system({nvim_prog, '-u', 'NONE', '-i', 'NONE', '--headless',
+                                          '-c', 'write test_empty_home', '+q'})
+    eq(false, tilde_in_cwd())
+  end
+
+  it("'~' folder not created in cwd if $HOME and related env not defined", function()
+    command("unlet $HOME")
+    write_and_test_tilde()
+
+    command("let $HOMEDRIVE='C:'")
+    command("let $USERPROFILE='C:\\'")
+    write_and_test_tilde()
+
+    command("unlet $HOMEDRIVE")
+    write_and_test_tilde()
+
+    command("unlet $USERPROFILE")
+    write_and_test_tilde()
+
+    command("let $HOME='%USERPROFILE%'")
+    command("let $USERPROFILE='C:\\'")
+    write_and_test_tilde()
+  end)
+
+  it("'~' folder not created in cwd if writing a file with invalid $HOME", function()
+    setenv('HOME', '/path/does/not/exist')
+    write_and_test_tilde()
+  end)
+
+  it("'~' folder not created in cwd if writing a file with $HOME=''", function()
+    command("let $HOME=''")
+    write_and_test_tilde()
+  end)
+end)