Build the fuzzers from oss-fuzz

Build the fuzzers from oss-fuzz

The fuzzers from oss-fuzz in projects/e2fsprogs/fuzz (as of commit
78ecd3f07fca with some slight modifications for better error
reporting) have been placed in the tests/fuzz directory and the
configure script now supports a new option --enable-fuzzing which will
build these fuzzers using clang's -fsanitize=fuzzer command line
option.

In general, some sanitizer such as --enable-addrsan or --enable-ubsan
(to enable ASAN or UBSAN, respectively) should be enabled alongside
--enable-fuzzing.

A typical configure command to build the fuzzers might be:

  configure CC=clang CXX=clang++ CFLAGS=-g --enable-fuzzing --enable-addrsan

Signed-off-by: Theodore Ts'o <tytso@mit.edu>