Fix ShellCheck warnings and add tests

[dbohdan]

Running ShellCheck on retry generates the following list of warnings. This PR fixes the warnings. It also adds tests, a Bash script that runs three basic functionality tests on retry.

In retry line 11:
    sleep_time=`awk "BEGIN {t = $min_sleep * $(( (1<<($attempts -1)) )); print (t > $max_sleep ? $max_sleep : t)}"`
               ^-- SC2006 (style): Use $(...) notation instead of legacy backticks `...`.
                                                      ^-------^ SC2004 (style): $/${} is unnecessary on arithmetic variables.

Did you mean: 
    sleep_time=$(awk "BEGIN {t = $min_sleep * $(( (1<<($attempts -1)) )); print (t > $max_sleep ? $max_sleep : t)}")

In retry line 43:
      sleep $sleep_time
            ^---------^ SC2086 (info): Double quote to prevent globbing and word splitting.

Did you mean: 
      sleep "$sleep_time"

In retry line 57:
      attempts=$[$attempts +1]
               ^-------------^ SC2007 (style): Use $((..)) instead of deprecated $[..]

In retry line 61:
  if [ $attempts -gt $max_tries ]; then
       ^-------^ SC2086 (info): Double quote to prevent globbing and word splitting.
                     ^--------^ SC2086 (info): Double quote to prevent globbing and word splitting.

Did you mean: 
  if [ "$attempts" -gt "$max_tries" ]; then

In retry line 64:
      eval $fail_script
           ^----------^ SC2086 (info): Double quote to prevent globbing and word splitting.

Did you mean: 
      eval "$fail_script"

In retry line 74:
if [ "$BASH_SOURCE" == "$0" ]; then
      ^----------^ SC2128 (warning): Expanding an array without an index only gives the first element.

In retry line 78:
    local retry=$(basename $0)
          ^---^ SC2155 (warning): Declare and assign separately to avoid masking return values.
                           ^-- SC2086 (info): Double quote to prevent globbing and word splitting.

Did you mean: 
    local retry=$(basename "$0")

In retry line 112:
  if [[ $? -ne 0 ]]; then
        ^-- SC2181 (style): Check exit code directly with e.g. 'if ! mycmd;', not indirectly with $?.

For more information:
  https://www.shellcheck.net/wiki/SC2128 -- Expanding an array without an ind...
  https://www.shellcheck.net/wiki/SC2155 -- Declare and assign separately to ...
  https://www.shellcheck.net/wiki/SC2086 -- Double quote to prevent globbing ...