Don't automatically adjust memory_limit to 2M

As PHP has a minimum memory usage of 2M (size of allocator chunk),
setting a limit below that value is not meaningful and will be
automatically rounded up to the chunk size. Rather than doing this
silently, show the newly introduced error message.

The memory limit had to be increased to 2M for a number of tests.

tests/lang/bug45392 has been marked as XFAIL. This old bugfix is
not working as intended. The memory limit in main's `PG(memory_limit)`
differs from the one in zend_alloc. In zend_alloc the `AG(mm_heap)->limit`
is defined as `max(passed_value, ZEND_MM_CHUNK_SIZE)`. The check made in
an unclean shutdown will never be true unless the memory limit is lower
than ZEND_MM_CHUNK_SIZE, which happened to be the case in the test.
https://bugs.php.net/bug.php?id=45392
https://github.com/php/php-src/commit/fcc0fdd125fdb9e1713f91d027fe07d680a0cf36

Zend/tests/fibers/out-of-memory-in-fiber.phpt

@@ -1,7 +1,7 @@
 --TEST--
 Out of Memory in a fiber
 --INI--
-memory_limit=10K
+memory_limit=2M
 --SKIPIF--
 <?php
 if (getenv("USE_ZEND_ALLOC") === "0") {

Zend/tests/fibers/out-of-memory-in-nested-fiber.phpt

@@ -1,7 +1,7 @@
 --TEST--
 Out of Memory in a nested fiber
 --INI--
-memory_limit=10K
+memory_limit=2M
 --SKIPIF--
 <?php
 if (getenv("USE_ZEND_ALLOC") === "0") {

Zend/tests/fibers/out-of-memory-in-recursive-fiber.phpt

@@ -1,7 +1,7 @@
 --TEST--
 Out of Memory from recursive fiber creation
 --INI--
-memory_limit=10K
+memory_limit=2M
 --SKIPIF--
 <?php
 if (getenv("USE_ZEND_ALLOC") === "0") {

Zend/zend_alloc.c

@@ -2661,9 +2661,6 @@ ZEND_API char* ZEND_FASTCALL zend_strndup(const char *s, size_t length)
 ZEND_API zend_result zend_set_memory_limit(size_t memory_limit)
 {
 #if ZEND_MM_LIMIT
-	if (memory_limit < ZEND_MM_CHUNK_SIZE) {
-		memory_limit = ZEND_MM_CHUNK_SIZE;
-	}
 	if (UNEXPECTED(memory_limit < AG(mm_heap)->real_size)) {
 		return FAILURE;
 	}

ext/standard/tests/streams/bug78902.phpt

@@ -1,14 +1,14 @@
 --TEST--
 Bug #78902: Memory leak when using stream_filter_append
 --INI--
-memory_limit=512k
+memory_limit=2M
 --FILE--
 <?php
 
 /** create temporary file 2mb file */
 $tmp_file_name = tempnam(sys_get_temp_dir(), 'test_');
 $fp = fopen($tmp_file_name, 'w+');
-$size = 1024 * 1024 * 2; // 2mb
+$size = 1024 * 1024 * 2; // 2mb, larger than the memory limit
 $chunk = 1024;
 while ($size > 0) {
     fputs($fp, str_pad('', min($chunk,$size)));

tests/lang/bug45392.phpt

@@ -2,6 +2,8 @@
 Bug #45392 (ob_start()/ob_end_clean() and memory_limit)
 --INI--
 display_errors=stderr
+--XFAIL--
+The issue has not yet been resolved.
 --SKIPIF--
 <?php
 if (getenv("USE_ZEND_ALLOC") === "0") {
@@ -10,7 +12,7 @@ if (getenv("USE_ZEND_ALLOC") === "0") {
 --FILE--
 <?php
 echo __LINE__ . "\n";
-ini_set('memory_limit', 100);
+ini_set('memory_limit', "2M");
 ob_start();
 $i = 0;
 while($i++ < 5000)  {