Move duplicate code in testNormalUpdateAvailable to a new method

Problem/Motivation

In #3100386: Create contrib update module test cases that use semantic versioning @dww pointed out that in this snippet from testNormalUpdateAvailable

switch ($minor_version) {
          case 0:
            // Both stable and unstable releases are available.
            // A stable release is the latest.
            if ($extra_version == '') {
              $this->assertUpdateTableTextNotContains('Up to date');
              $this->assertUpdateTableTextContains('Update available');
              $this->assertVersionUpdateLinks('Recommended version:', $full_version);
              $this->assertUpdateTableTextNotContains('Latest version:');
              $this->assertUpdateTableElementContains('warning.svg');
            }
            // Only unstable releases are available.
            // An unstable release is the latest.
            else {
              $this->assertUpdateTableTextContains('Up to date');
              $this->assertUpdateTableTextNotContains('Update available');
              $this->assertUpdateTableTextNotContains('Recommended version:');
              $this->assertVersionUpdateLinks('Latest version:', $full_version);
              $this->assertUpdateTableElementContains('check.svg');
            }
            break;

          case 1:
            // Both stable and unstable releases are available.
            // A stable release is the latest.
            if ($extra_version == '') {
              $this->assertUpdateTableTextNotContains('Up to date');
              $this->assertUpdateTableTextContains('Update available');
              $this->assertVersionUpdateLinks('Recommended version:', $full_version);
              $this->assertUpdateTableTextNotContains('Latest version:');
              $this->assertUpdateTableElementContains('warning.svg');
            }
            // Both stable and unstable releases are available.
            // An unstable release is the latest.
            else {
              $this->assertUpdateTableTextNotContains('Up to date');
              $this->assertUpdateTableTextContains('Update available');
              $this->assertVersionUpdateLinks('Recommended version:', '8.1.0');
              $this->assertVersionUpdateLinks('Latest version:', $full_version);
              $this->assertUpdateTableElementContains('warning.svg');
            }
            break;
        }

The portion

if ($extra_version == '') {
              $this->assertUpdateTableTextNotContains('Up to date');
              $this->assertUpdateTableTextContains('Update available');
              $this->assertVersionUpdateLinks('Recommended version:', $full_version);
              $this->assertUpdateTableTextNotContains('Latest version:');
              $this->assertUpdateTableElementContains('warning.svg');
            }

Is a duplicates and could be moved out of the switch.
Since code was just being copied in that issue it was out of scope.

Proposed resolution

Move duplicate to a method

Remaining tasks

User interface changes

None

API changes

none

Data model changes

none

Release notes snippet

none