tests: add GITTEST_SLOW env var check

Writing very large files may be slow, particularly on inefficient
filesystems and when running instrumented code to detect invalid memory
accesses (eg within valgrind or similar tools).

Introduce `GITTEST_SLOW` so that tests that are slow can be skipped by
the CI system.

tests/odb/largefiles.c

@@ -45,10 +45,9 @@ void test_odb_largefiles__write_from_memory(void)
 	cl_skip();
 #endif
 
-	if (!cl_is_env_set("GITTEST_INVASIVE_FS_SIZE"))
-		cl_skip();
-
-	if (!cl_is_env_set("GITTEST_INVASIVE_MEMORY"))
+	if (!cl_is_env_set("GITTEST_INVASIVE_FS_SIZE") ||
+		!cl_is_env_set("GITTEST_INVASIVE_MEMORY") ||
+		!cl_is_env_set("GITTEST_SLOW"))
 		cl_skip();
 
 	for (i = 0; i < (3041*126103); i++)
@@ -64,7 +63,8 @@ void test_odb_largefiles__streamwrite(void)
 {
 	git_oid expected, oid;
 
-	if (!cl_is_env_set("GITTEST_INVASIVE_FS_SIZE"))
+	if (!cl_is_env_set("GITTEST_INVASIVE_FS_SIZE") ||
+		!cl_is_env_set("GITTEST_SLOW"))
 		cl_skip();
 
 	git_oid_fromstr(&expected, "3fb56989cca483b21ba7cb0a6edb229d10e1c26c");
@@ -82,10 +82,9 @@ void test_odb_largefiles__read_into_memory(void)
 	cl_skip();
 #endif
 
-	if (!cl_is_env_set("GITTEST_INVASIVE_FS_SIZE"))
-		cl_skip();
-
-	if (!cl_is_env_set("GITTEST_INVASIVE_MEMORY"))
+	if (!cl_is_env_set("GITTEST_INVASIVE_FS_SIZE") ||
+		!cl_is_env_set("GITTEST_INVASIVE_MEMORY") ||
+		!cl_is_env_set("GITTEST_SLOW"))
 		cl_skip();
 
 	writefile(&oid);
@@ -103,10 +102,9 @@ void test_odb_largefiles__read_into_memory_rejected_on_32bit(void)
 	cl_skip();
 #endif
 
-	if (!cl_is_env_set("GITTEST_INVASIVE_FS_SIZE"))
-		cl_skip();
-
-	if (!cl_is_env_set("GITTEST_INVASIVE_MEMORY"))
+	if (!cl_is_env_set("GITTEST_INVASIVE_FS_SIZE") ||
+		!cl_is_env_set("GITTEST_INVASIVE_MEMORY") ||
+		!cl_is_env_set("GITTEST_SLOW"))
 		cl_skip();
 
 	writefile(&oid);