PHP 8.1: prevent a "null to non-nullable" deprecation notice (test only fix)
PHP 8.1: prevent a "null to non-nullable" deprecation notice (test only fix)

Not all tests in the `InstallerTest` class actually create a temporary directory and set the `$this->tempComposerHome` property.

Those tests which didn't, throw a notice in PHP 8.1.

Fixes 3 notices along the lines of:
```
Deprecation triggered by Composer\Test\InstallerTest::tearDown:
is_dir(): Passing null to parameter #1 ($filename) of type string is deprecated

Stack trace:
0 [internal function]: Symfony\Bridge\PhpUnit\DeprecationErrorHandler->handleError(8192, '...', '...', 53)
1 tests/Composer/Test/InstallerTest.php(53): is_dir(NULL)
...
```

johnstevenson/composer,stof/composer,nicolas-grekas/composer,alcohol/composer,staabm/composer,stefangr/composer,milleruk/composer,composer/composer