Added reentrant and gdb testing mechanisms to test framework

Aside from reworking the internals of test_.py to work well with
inherited TestCase classes, this also provides the two main features
that were the main reason for revamping the test framework

1. ./scripts/test_.py --reentrant

   Runs reentrant tests (tests with reentrant=true in the .toml
   configuration) under gdb such that the program is killed on every
   call to lfs_emubd_prog or lfs_emubd_erase.

   Currently this just increments a number of prog/erases to skip, which
   means it doesn't necessarily check every possible branch of the test,
   but this should still provide a good coverage of power-loss tests.

2. ./scripts/test_.py --gdb

   Run the tests and if a failure is hit, drop into GDB. In theory this
   will be very useful for reproducing and debugging test failures.

   Note this can be combined with --reentrant to drop into GDB on the
   exact cycle of power-loss where the tests fail.

tests_/test_dirs.toml

@@ -10,6 +10,17 @@ code = """
     lfs_unmount(&lfs) => 0;
 """
 
+[[case]] # reentrant format
+code = """
+    int err = lfs_mount(&lfs, &cfg);
+    if (err) {
+        lfs_format(&lfs, &cfg) => 0;
+        lfs_mount(&lfs, &cfg) => 0;
+    }
+    lfs_unmount(&lfs) => 0;
+"""
+reentrant = true
+
 [[case]] # root
 code = """
     lfs_format(&lfs, &cfg) => 0;
@@ -53,7 +64,7 @@ code = """
     }
     lfs_dir_read(&lfs, &dir, &info) => 0;
     lfs_dir_close(&lfs, &dir) => 0;
-    lfs_unmount(&lfs);
+    lfs_unmount(&lfs) => 0;
 """
 define.N = 'range(0, 100, 3)'