Adding tests for conversion of string percentage to numeric

Adding two test cases:
1) Test the function in `StringHelper.php`.  Modeled this test on the test for the `convertToNumberIfFraction` function.
2) Test a spreadsheet with a string percentage in a formula to see if it  calculates the formula correctly.

tests/PhpSpreadsheetTests/Calculation/CalculationTest.php

@@ -180,6 +180,18 @@ class CalculationTest extends TestCase
         self::assertEquals('9', $cell3->getCalculatedValue());
     }
 
+    public function testCellWithStringPercentage(): void
+    {
+        $spreadsheet = new Spreadsheet();
+        $workSheet = $spreadsheet->getActiveSheet();
+        $cell1 = $workSheet->getCell('A1');
+        $cell1->setValue('2%');
+        $cell2 = $workSheet->getCell('B1');
+        $cell2->setValue('=100*A1');
+
+        self::assertEquals('2', $cell2->getCalculatedValue());
+    }
+
     public function testBranchPruningFormulaParsingSimpleCase(): void
     {
         $calculation = Calculation::getInstance();

tests/PhpSpreadsheetTests/Shared/StringHelperTest.php

@@ -148,4 +148,33 @@ class StringHelperTest extends TestCase
             'improper fraction' => ['1.75', '7/4'],
         ];
     }
+
+    /**
+     * @dataProvider providerPercentages
+     */
+    public function testPercentage(string $expected, string $value): void
+    {
+        $originalValue = $value;
+        $result = StringHelper::convertToNumberIfPercent($value);
+        if ($result === false) {
+            self::assertSame($expected, $originalValue);
+            self::assertSame($expected, $value);
+        } else {
+            self::assertSame($expected, (string) $value);
+            self::assertNotEquals($value, $originalValue);
+        }
+    }
+
+    public function providerPercentages(): array
+    {
+        return [
+            'non-percentage' => ['10', '10'],
+            'single digit percentage' => ['0.02', '2%'],
+            'two digit percentage' => ['0.13', '13%'],
+            'negative single digit percentage' => ['-0.07', '-7%'],
+            'negative two digit percentage' => ['-0.75', '-75%'],
+            'large percentage' => ['98.45', '9845%'],
+            'small percentage' => ['0.0005', '0.05%'],
+        ];
+    }
 }