Introduce pedantic mode for tester

Motivation
----------
External tools need to know whether or not any test in suite failed. For
example buildbot, or packaging tool would like to stop the execution and
mark it as failed if some tests does not pass.

Modification
------------
Add `--pedantic` switch to tester program which will force it to quit
with non-zero exit code if at least one failure detected. Also update
`tests()` proc in koch to inspect result from tester and propagate it to
user.

Result
------
Nothing has changed in default behaviour. But following invocations will
exit with non-zero code if there failed tests:

    ./koch tests --pedantic all
    ./tests/testament/tester --pedantic all

1 files changed, 3 insertions, 1 deletions

diff --git a/koch.nim b/koch.nim
index 55019b544..a951b422f 100644
--- a/koch.nim
+++ b/koch.nim
@@ -333,8 +333,10 @@ proc tests(args: string) =
   # taint mode test :-)
   exec "nim cc --taintMode:on tests/testament/tester"
   let tester = quoteShell(getCurrentDir() / "tests/testament/tester".exe)
-  exec tester & " " & (args|"all")
+  let success = tryExec tester & " " & (args|"all")
   exec tester & " html"
+  if not success:
+    quit("tests failed", QuitFailure)
 
 proc temp(args: string) =
   var output = "compiler" / "nim".exe