tests: replace /bin/bash with bash

For portability.

Author: bjornfor

test/functests/test_exclude_mono.sh

@@ -66,7 +66,7 @@ get_monotonic_time()
     clock_id=$1; shift;
     FAKETIME_DONT_FAKE_MONOTONIC=${dont_fake_mono} \
     fakecmd "2014-07-21 09:00:00" \
-    /bin/bash -c "for i in 1 2; do \
+    bash -c "for i in 1 2; do \
     perl -w -MTime::HiRes=clock_gettime,${clock_id} -E \
     'say clock_gettime(${clock_id})'; \
     sleep 1; \