patch 8.1.1305: there is no easy way to manipulate environment variables

Problem:    There is no easy way to manipulate environment variables.
Solution:   Add environ(), getenv() and setenv(). (Yasuhiro Matsumoto,
            closes #2875)

src/testdir/test_environ.vim

@@ -0,0 +1,44 @@
+scriptencoding utf-8
+
+func Test_environ()
+  unlet! $TESTENV
+  call assert_equal(0, has_key(environ(), 'TESTENV'))
+  let $TESTENV = 'foo'
+  call assert_equal(1, has_key(environ(), 'TESTENV'))
+  let $TESTENV = 'こんにちわ'
+  call assert_equal('こんにちわ', environ()['TESTENV'])
+endfunc
+
+func Test_getenv()
+  unlet! $TESTENV
+  call assert_equal(v:null, getenv('TESTENV'))
+  let $TESTENV = 'foo'
+  call assert_equal('foo', getenv('TESTENV'))
+endfunc
+
+func Test_setenv()
+  unlet! $TESTENV
+  call setenv('TEST ENV', 'foo')
+  call assert_equal('foo', getenv('TEST ENV'))
+  call setenv('TEST ENV', v:null)
+  call assert_equal(v:null, getenv('TEST ENV'))
+endfunc
+
+func Test_external_env()
+  call setenv('FOO', 'HelloWorld')
+  if has('win32')
+    let result = system('echo %FOO%')
+  else
+    let result = system('echo $FOO')
+  endif
+  let result = substitute(result, '[ \r\n]', '', 'g')
+  call assert_equal('HelloWorld', result)
+
+  call setenv('FOO', v:null)
+  if has('win32')
+    let result = system('set | grep ^FOO=')
+  else
+    let result = system('env | grep ^FOO=')
+  endif
+  call assert_equal('', result)
+endfunc