Fix mktemp portability issue affecting test coverage. (addresses #752)

Also update Makefiles to behave better for brief targets and in absence
of external test repos.

testing/Makefile

@@ -12,8 +12,8 @@ make-brief:
 	@for repo in $(DIRS); do (cd $$repo && make brief ); done
 
 coverage:
-	@cp btest/coverage.log `mktemp brocov.tmp.XXX`
-	@for f in external/*/coverage.log; do cp $$f `mktemp brocov.tmp.XXX`; done
+	@test -f btest/coverage.log && cp btest/coverage.log `mktemp brocov.tmp.XXX` || true
+	@for f in external/*/coverage.log; do test -f $$f && cp $$f `mktemp brocov.tmp.XXX` || true; done
 	@echo "Complete test suite code coverage:"
 	@./scripts/coverage-calc "brocov.tmp.*" coverage.log `pwd`/../scripts
 	@rm -f brocov.tmp.*

testing/btest/Makefile

@@ -2,14 +2,16 @@
 DIAG=diag.log
 BTEST=../../aux/btest/btest
 
-all:
+all: cleanup
     # Showing all tests.
-	@rm -f $(DIAG)
-	@rm -f .tmp/script-coverage*
 	@$(BTEST) -f $(DIAG)
 	@../scripts/coverage-calc ".tmp/script-coverage*" coverage.log `pwd`/../../scripts
 
-brief:
+brief: cleanup
     # Brief output showing only failed tests.
-	@rm -f $(DIAG)
 	@$(BTEST) -b -f $(DIAG)
+	@../scripts/coverage-calc ".tmp/script-coverage*" coverage.log `pwd`/../../scripts
+
+cleanup:
+	@rm -f $(DIAG)
+	@rm -f .tmp/script-coverage*

testing/scripts/btest-bg-run

@@ -4,4 +4,4 @@
 # when collecting Bro script coverage statistics so that two independent
 # Bro processing don't try to write those usage statistics to the same file.
 
-BRO_PROFILER_FILE=`mktemp -t script-coverage` $BTEST_PATH/btest-bg-run $@
+BRO_PROFILER_FILE=`mktemp $TMPDIR/script-coverage.XXXX` $BTEST_PATH/btest-bg-run $@