Make bin_test semi-independent re: pylib and build context.

Should address #743 (closed) and #793 (closed).

following ./waf configure --alltests --enable-pylib=none (output slightly simplified)

> ./waf build -pj12                             
--- building host --- 
Waf: Entering directory `/Users/jamesb/scm/ntpsec/build/host'
[5/5][100%][|][=======================================================>][0.273s]164 keywords consumed 711 states of 2047 max.

[5/5][100%][/][=======================================================>][0.289s]
Waf: Leaving directory `/Users/jamesb/scm/ntpsec/build/host'
--- building main --- 
Waf: Entering directory `/Users/jamesb/scm/ntpsec/build/main'
--- PYTHONPATH is not set, loading the Python ntp library may be troublesome ---
[330/330][100%][|][===================================================>][3.806s]
Waf: Leaving directory `/Users/jamesb/scm/ntpsec/build/main'
Wrote test log to:  /Users/jamesb/scm/ntpsec/build/main/test.log
execution summary 
  tests that pass 3/3 
    /Users/jamesb/scm/ntpsec/build/main/libaes_siv/runtests 
    /Users/jamesb/scm/ntpsec/build/main/tests/test_ntpd 
    /Users/jamesb/scm/ntpsec/build/main/tests/test_libntp 
  tests that fail 0/3 
running: main/ntpclients/ntpleapfetch --version  OK 
running: main/ntpd/ntpd --version  OK 
running: main/ntpfrob/ntpfrob -V  OK 
running: main/ntptime/ntptime -V  OK 
'build' finished successfully (4.135s)
> ./waf bin_test -v                             
running: main/ntpclients/ntpleapfetch --version  OK 
running: main/ntpd/ntpd --version  OK 
running: main/ntpfrob/ntpfrob -V  OK 
running: main/ntptime/ntptime -V  OK 
'bin_test' finished successfully (0.042s)
> 

following ./waf configure --alltests --enable-pylib=ext or ./waf configure --alltests --enable-pylib=ffi

> ./waf build -pj12
--- building host --- 
Waf: Entering directory `/Users/jamesb/scm/ntpsec/build/host'
[5/5][100%][|][=======================================================>][0.028s]
Waf: Leaving directory `/Users/jamesb/scm/ntpsec/build/host'
--- building main --- 
Waf: Entering directory `/Users/jamesb/scm/ntpsec/build/main'
--- PYTHONPATH is not set, loading the Python ntp library may be troublesome ---
[416/416][100%][\][===================================================>][0.580s]
Waf: Leaving directory `/Users/jamesb/scm/ntpsec/build/main'
Wrote test log to:  /Users/jamesb/scm/ntpsec/build/main/test.log
execution summary 
  tests that pass 6/6 
    /Users/jamesb/scm/ntpsec/build/main/tests/pylib/test_ntpc.py 
    /Users/jamesb/scm/ntpsec/build/main/tests/pylib/test_statfiles.py 
    /Users/jamesb/scm/ntpsec/build/main/tests/pylib/test_agentx.py 
    /Users/jamesb/scm/ntpsec/build/main/tests/pylib/test_packet.py 
    /Users/jamesb/scm/ntpsec/build/main/tests/pylib/test_agentx_packet.py 
    /Users/jamesb/scm/ntpsec/build/main/tests/pylib/test_util.py 
  tests that fail 0/6 
running: main/ntpclients/ntpdig --version  OK 
running: main/ntpclients/ntpkeygen --version  OK 
running: main/ntpclients/ntplogtemp --version  OK 
running: main/ntpclients/ntpmon --version  OK 
running: main/ntpclients/ntpq --version  OK 
running: main/ntpclients/ntpsnmpd --version  OK 
running: main/ntpclients/ntpsweep --version  OK 
running: main/ntpclients/ntptrace --version  OK 
running: main/ntpclients/ntpviz --version  OK 
running: main/ntpclients/ntpwait --version  OK 
running: main/ntpclients/ntpleapfetch --version  OK 
running: main/ntpd/ntpd --version  OK 
running: main/ntpfrob/ntpfrob -V  OK 
running: main/ntptime/ntptime -V  OK 
'build' finished successfully (1.229s)
> ./waf bin_test -v
running: main/ntpclients/ntpleapfetch --version  OK 
running: main/ntpd/ntpd --version  OK 
running: main/ntpfrob/ntpfrob -V  OK 
running: main/ntptime/ntptime -V  OK 
'bin_test' finished successfully (0.031s)
>