Fix // in some paths and add regression test

Fixes trailing / on TMPDIR for MacOS (fixes #605)

libexec/bats-core/bats-exec-test

diff --git a/libexec/bats-core/bats-exec-test b/libexec/bats-core/bats-exec-test
index 241a5cf..0483916 100755
--- a/libexec/bats-core/bats-exec-test
+++ b/libexec/bats-core/bats-exec-test
@@ -70,7 +70,7 @@
 fi
 
 bats_create_test_tmpdirs() {
-  local tests_tmpdir="${BATS_RUN_TMPDIR}/test/"
+  local tests_tmpdir="${BATS_RUN_TMPDIR}/test"
   if ! mkdir -p "$tests_tmpdir"; then
     printf 'Failed to create: %s\n' "$tests_tmpdir" >&2
     exit 1