Add Unit-Tests

Add unittests via shunit2 (https://github.com/kward/shunit2),
that integrate with travis-ci.org.

.gitmodules

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+[submodule "shunit2"]
+	path = shunit2
+	url = git@github.com:kward/shunit2.git

.travis.yml

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+language: php
+sudo: false
+
+php:
+    - '7.0'
+
+script:
+    - test/powerlevel9k.spec
+

shunit2

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+Subproject commit 60dd60bcd1573befe38465010263ab242e55811d

test/powerlevel9k.spec

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+#!/usr/bin/env zsh
+#vim:ft=zsh ts=2 sw=2 sts=2 et fenc=utf-8
+
+# Required for shunit2 to run correctly
+setopt shwordsplit
+SHUNIT_PARENT=$0
+
+function oneTimeSetUp() {
+  # Load Powerlevel9k
+  source powerlevel9k.zsh-theme
+  source functions/*
+}
+
+function testJoinedSegments() {
+  POWERLEVEL9K_LEFT_PROMPT_ELEMENTS=(dir dir_joined)
+
+  assertEquals "%K{blue} %F{black}%~ %K{blue}%F{black}%F{black}%~ %k%F{blue}%f " "$(build_left_prompt)"
+
+  unset POWERLEVEL9K_LEFT_PROMPT_ELEMENTS
+}
+
+function testTransitiveJoinedSegments() {
+  POWERLEVEL9K_LEFT_PROMPT_ELEMENTS=(dir root_indicator_joined dir_joined)
+
+  assertEquals "%K{blue} %F{black}%~ %K{blue}%F{black}%F{black}%~ %k%F{blue}%f " "$(build_left_prompt)"
+
+  unset POWERLEVEL9K_LEFT_PROMPT_ELEMENTS
+}
+
+function testJoiningWithConditionalSegment() {
+  POWERLEVEL9K_LEFT_PROMPT_ELEMENTS=(dir background_jobs dir_joined)
+
+  assertEquals "%K{blue} %F{black}%~ %K{blue}%F{black} %F{black}%~ %k%F{blue}%f " "$(build_left_prompt)"
+
+  unset POWERLEVEL9K_LEFT_PROMPT_ELEMENTS
+}
+
+function testDynamicColoringOfSegmentsWork() {
+  POWERLEVEL9K_LEFT_PROMPT_ELEMENTS=(dir)
+  POWERLEVEL9K_DIR_HOME_SUBFOLDER_BACKGROUND='red'
+
+  assertEquals "%K{red} %F{black}%~ %k%F{red}%f " "$(build_left_prompt)"
+
+  unset POWERLEVEL9K_LEFT_PROMPT_ELEMENTS
+  unset POWERLEVEL9K_DIR_HOME_SUBFOLDER_BACKGROUND
+}
+
+source shunit2/source/2.1/src/shunit2