TextData Coverage and Minor Bug Fixes (#1744)

This had been intended to get 100% coverage for TextData functions, and it does that.
However, some minor bugs requiring source changes arose during testing.
- the Excel CHAR function restricts its argument to 1-255. PhpSpreadsheet CHARACTER
  had been allowing 0+. Also, there is no need to test if iconv exists,
  since it is part of Composer requirements.
- The DOLLAR function had been returning NUM for invalid arguments. Excel returns VALUE.
  Also, negative amounts were not being handled correctly.
- The FIXEDFORMAT function had been returning NUM for invalid arguments. Excel FIXED returns VALUE.

tests/data/Calculation/TextData/FIXED.php

@@ -20,13 +20,41 @@ return [
         true,
     ],
     [
-        '#NUM!',
+        '-123456.79',
+        -123456.789,
+        2,
+        true,
+    ],
+    [
+        '123500',
+        123456.789,
+        -2,
+        true,
+    ],
+    [
+        '123,500',
+        123456.789,
+        -2,
+    ],
+    [
+        '-123500',
+        -123456.789,
+        -2,
+        true,
+    ],
+    [
+        '-123,500',
+        -123456.789,
+        -2,
+    ],
+    [
+        '#VALUE!',
         'ABC',
         2,
         null,
     ],
     [
-        '#NUM!',
+        '#VALUE!',
         123.456,
         'ABC',
         null,