test: check for debug symbols in notmuch

In the future, tests may rely on debug symbols being present in
notmuch, so we plan to switch the default flags.

The main purpose of this test is to help explain the perhaps
mysterious failures of other tests which rely on symbols being
present.
configure: add debug flags by default.

This makes development (in particular the test suite) easier. Those
concerned about the extra diskspace can override the default or use
strip.
test: Port atomicity test to Python

Previously, this was implemented using a horrible GDB script (because
there is no such thing as a non-horrible GDB script).  This GDB script
often broke with newer versions of GDB for mysterious reasons.  Port
the test script to GDB's Python API, which makes the code much cleaner
and, hopefully, more stable.
hex-escape: remove unused variable default_buf_size

Found by clang:

CC  -O2 util/hex-escape.o
util/hex-escape.c:28:21: warning: unused variable 'default_buf_size'
      [-Wunused-const-variable]
static const size_t default_buf_size = 1024;
                    ^
1 warning generated.