fixes test in src/lib/test-file-cache.c for musl

--- a/src/lib/test-file-cache.c	2021-01-04 17:55:39.550032767 +0000
+++ b/src/lib/test-file-cache.c	2021-01-04 17:54:31.439645416 +0000
@@ -263,7 +263,7 @@
 	};
 	const char *errstr =
 		t_strdup_printf("mmap_anon(.test_file_cache, %zu) failed: "
-				"Cannot allocate memory", page_size);
+				"Out of memory", page_size);
 	test_assert(setrlimit(RLIMIT_AS, &rl_new) == 0);
 	test_expect_error_string(errstr);
 	test_assert(file_cache_set_size(cache, 1024) == -1);
@@ -271,7 +271,7 @@
 
 	/* same for mremap */
 	errstr = t_strdup_printf("mremap_anon(.test_file_cache, %zu) failed: "
-				 "Cannot allocate memory", page_size*2);
+				 "Out of memory", page_size*2);
 	test_assert(file_cache_set_size(cache, 1) == 0);
 	test_assert(setrlimit(RLIMIT_AS, &rl_new) == 0);
 	test_expect_error_string(errstr);
