bin(run-tests): improve status messages (#305)

Changes:

- Print the exercise name in its canonical kebab-case form, not its
  snake_case form.

- Improve ellipsis usage.

- Prefix error messages with Error, being consistent with the style in
  a recent commit [1].

- Add quotes around a path.

- Consistently use "directory", not "folder".

- Consistently write status messages to stderr.

Note that the `zig test` output goes to stderr. Before this commit:

    $ bin/run-tests > /dev/null
    All 9 tests passed.
    All 50 tests passed.
    All 13 tests passed.
    [...]

With this commit:

    $ bin/run-tests > /dev/null
    Running tests for acronym...
    All 9 tests passed.

    Running tests for allergies...
    All 50 tests passed.

    Running tests for armstrong-numbers...
    All 13 tests passed.

    [...]

[1] 7ff0372, 2023-08-23, "bin(run-tests): error for missing example"

bin/run-tests

@@ -8,14 +8,14 @@ execute_test() {
     local exercise_name
     exercise_name=$(echo "$1" | tr '-' '_')
 
-    printf '%s\n' "Running tests for ${exercise_name}"
+    printf '%s\n' "Running tests for ${1}..." >&2
 
     # Copy the examples with the correct name for the exercise
     if [[ -f "./.meta/example.zig" ]]; then
       mv ./.meta/example.zig "${exercise_name}".zig
       # No building required, just test it
       zig test "test_${exercise_name}.zig"
-      printf '\n'
+      printf '\n' >&2
     else
       printf '%s\n' "Error: ${exercise_name} lacks its '.meta/example.zig' file" >&2
       exit 1
@@ -30,7 +30,7 @@ main() {
   cd "${script_dir}"/..
 
   if [[ ! -d "exercises/practice" ]]; then
-    printf '%s\n' "The exercises/practice folder is missing..." >&2
+    printf '%s\n' "Error: the 'exercises/practice' directory is missing" >&2
     exit 1
   fi
 
@@ -47,7 +47,7 @@ main() {
       local exercises=("$@")
       for exercise in "${exercises[@]}"; do
         if [[ ! -d "${exercise}" ]]; then
-          printf '%s\n' "The specified exercise '${1}' does not exist" >&2
+          printf '%s\n' "Error: the specified exercise '${1}' does not exist" >&2
           exit 1
         fi
       done