patch 8.1.2342: random number generator in Vim script is slow

Problem:    Random number generator in Vim script is slow.
Solution:   Add rand() and srand(). (Yasuhiro Matsumoto, closes #1277)

src/testdir/test_random.vim

@@ -0,0 +1,28 @@
+" Tests for srand() and rand()
+
+func Test_Rand()
+  let r = srand(123456789)
+  call assert_equal([123456789, 362436069, 521288629, 88675123], r)
+  call assert_equal(3701687786, rand(r))
+  call assert_equal(458299110, rand(r))
+  call assert_equal(2500872618, rand(r))
+  call assert_equal(3633119408, rand(r))
+  call assert_equal(516391518, rand(r))
+
+  call test_settime(12341234)
+  let s = srand()
+  call assert_equal(s, srand())
+  call test_settime(12341235)
+  call assert_notequal(s, srand())
+
+  call srand()
+  let v = rand()
+  call assert_notequal(v, rand())
+
+  call assert_fails('echo srand([1])', 'E745:')
+  call assert_fails('echo rand([1, 2, 3])', 'E475:')
+  call assert_fails('echo rand([[1], 2, 3, 4])', 'E475:')
+  call assert_fails('echo rand([1, [2], 3, 4])', 'E475:')
+  call assert_fails('echo rand([1, 2, [3], 4])', 'E475:')
+  call assert_fails('echo rand([1, 2, 3, [4]])', 'E475:')
+endfunc