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.

scripts/explode_asserts.py

@@ -16,7 +16,8 @@ ASSERT_TESTS = {
             printf("%s:%d:assert: "
                 "assert failed with %"PRIiMAX", expected {comp} %"PRIiMAX"\\n",
                 {file}, {line}, (intmax_t)_lh, (intmax_t)_rh);
-            exit(-2);
+            fflush(NULL);
+            raise(SIGABRT);
         }}
     """,
     'str': """
@@ -26,7 +27,8 @@ ASSERT_TESTS = {
             printf("%s:%d:assert: "
                 "assert failed with \\\"%s\\\", expected {comp} \\\"%s\\\"\\n",
                 {file}, {line}, _lh, _rh);
-            exit(-2);
+            fflush(NULL);
+            raise(SIGABRT);
         }}
     """,
     'bool': """
@@ -36,7 +38,8 @@ ASSERT_TESTS = {
             printf("%s:%d:assert: "
                 "assert failed with %s, expected {comp} %s\\n",
                 {file}, {line}, _lh ? "true" : "false", _rh ? "true" : "false");
-            exit(-2);
+            fflush(NULL);
+            raise(SIGABRT);
         }}
     """,
 }
@@ -180,6 +183,7 @@ def main(args):
     outf.write("#include <stdbool.h>\n")
     outf.write("#include <stdint.h>\n")
     outf.write("#include <inttypes.h>\n")
+    outf.write("#include <signal.h>\n")
     outf.write(mkdecl('int',  'eq', '=='))
     outf.write(mkdecl('int',  'ne', '!='))
     outf.write(mkdecl('int',  'lt', '<'))