Tests: don't set memory limit below 1 MB

This wasn't an issue in dev, but it was breaking in CI with the error:
```
Failed to set memory limit to 1048576 bytes
```

I'm guessing the problem is related to the backwards-incompatible change
in PHP 7.4:
https://php.watch/versions/7.4/memory_limit-lt-2m
php/php-src#7040

Author: davidrans

SoftMemoryLimit/tests/MemoryCheckerTest.php

@@ -19,10 +19,10 @@ public function testInitializedMemoryChecker() {
     }
 
     public function testGetHardLimit() {
-        $memoryLimit = 1024 * 1024;
+        $memoryLimit = 3 * 1024 * 1024;
         $memoryChecker = \SoftMemoryLimit\MemoryChecker::getSingleton();
 
-        // Set it to a MB or something.
+        // Set it to about 3 MB.
         ini_set('memory_limit', "$memoryLimit");
         $this->assertSame($memoryLimit, $memoryChecker->getHardLimit());
     }