test-gdb: bring back to life

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

userland/gdb_tests/README.adoc

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+https://github.com/cirosantilli/linux-kernel-module-cheat#gdb-tests

userland/gdb_tests/add.c

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+#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/gdb_tests/add.py

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+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