test-gdb: bring back to life

Well factored with ./test-baremetal, allowing for target selection.

userland/c/add.c

@@ -1,13 +0,0 @@
-#include <assert.h>
-
-int main(void) {
-    int i, j, k;
-    i = 1;
-    /* test-gdb-op1 */
-    j = 2;
-    /* test-gdb-op2 */
-    k = i + j;
-    /* test-gdb-result */
-    if (k != 3)
-        assert(0);
-}

userland/c/add.py

@@ -1,9 +0,0 @@
-def test(self):
-    self.sendline('tbreak main')
-    self.sendline('continue')
-    self.continue_to('op1')
-    assert self.get_int('i') == 1
-    self.continue_to('op2')
-    assert self.get_int('j') == 2
-    self.continue_to('result')
-    assert self.get_int('k') == 3