Revert sloppy workaround in dabde0bc5, addressed by prev. commit

Revert sloppy workaround in dabde0bc5, addressed by prev. commit

Tighten up the Firebird setup, add an explicit travis-brewed 5.10 smoke and
add extra randomness to the testing order in t/750firebird.t

Tested to bits with:

--- a/maint/travis-ci_scripts/40_script.bash
+++ b/maint/travis-ci_scripts/40_script.bash
@@ -28,15 +28,17 @@ if [[ "$CLEANTEST" = "true" ]] ; then
   run_or_err "Prepare blib" "make pure_all"
   run_harness_tests
 else
-  PROVECMD="prove -lrswj$VCPU_USE xt t"
+  PROVECMD="prove -lrswj$VCPU_USE t/750firebird.t"

   # FIXME - temporary, until Package::Stash is fixed
   if perl -M5.010 -e 1 &>/dev/null ; then
     PROVECMD="$PROVECMD -T"
   fi

+  for f in $(seq 200) ; do
     echo_err "$(tstamp) running tests with \`$PROVECMD\`"
     $PROVECMD 2> >(tee "$TEST_STDERR_LOG")
+  done
 fi
 TEST_T1=$SECONDS

Results at: https://travis-ci.org/ribasushi/dbix-class/builds/100894293