[core] fix li_base64_dec() on whitespace

(regression in lighttpd 1.4.60 - lighttpd 1.4.65)

impacted: mod_wolfssl loading certificate private keys

must ignore whitespace in count of encoded chars

[tests] combine tests into test_common.c

combine common components tests into test_common.c

combine into a single executable to reduce compilation/link redundancy

[core] base64 encode round-up for required space

no need for extra work for precision allocation to avoid 0-3 extra chars

note: callers passing precise buffer size for without padding will need
t

[core] base64 encode round-up for required space

no need for extra work for precision allocation to avoid 0-3 extra chars

note: callers passing precise buffer size for without padding will need
to be modified to pass a slightly larger buffer, e.g. mod_secdownload

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[tests] test_base64.c clear buf vs reset

(can be slightly faster to clear buffer than to reset buffer)

[tests] use buffer_eq_slen() for str comparison

(substitute buffer_eq_slen() for buffer_is_equal_string())

[tests] move src/test_*.c to src/t/