tests: add shellspec spec file

spec/minishell_spec.sh

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+It "echos output"
+  Data
+    #|hello
+    #|there
+  End
+
+  When call ./minishell
+  The output line 1 should eq "hello"
+  The output line 2 should eq "hello"
+  The output line 3 should eq "there"
+  The output line 4 should eq "there"
+  The output line 5 should be undefined
+End

spec/spec_helper.sh

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+# shellcheck shell=sh
+
+# Defining variables and functions here will affect all specfiles.
+# Change shell options inside a function may cause different behavior,
+# so it is better to set them here.
+# set -eu
+
+# This callback function will be invoked only once before loading specfiles.
+spec_helper_precheck() {
+  # Available functions: info, warn, error, abort, setenv, unsetenv
+  # Available variables: VERSION, SHELL_TYPE, SHELL_VERSION
+  : minimum_version "0.28.1"
+}
+
+# This callback function will be invoked after a specfile has been loaded.
+spec_helper_loaded() {
+  :
+}
+
+# This callback function will be invoked after core modules has been loaded.
+spec_helper_configure() {
+  # Available functions: import, before_each, after_each, before_all, after_all
+  : import 'support/custom_matcher'
+}