test: add page

pages/common/test.md

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+# test
+
+> Evaluate condition.
+> If it is true, returns 0 exit status, otherwise returns 1.
+
+- Test if given variable is equal to given string
+
+`test $MY_VAR == '/bin/zsh'`
+
+- Test if given variable is empty
+
+`test -z $GIT_BRANCH`
+
+- Test if file exists
+
+`test -e {{filename}}`
+
+- Test if directory not exists
+
+`test ! -d {{path/to/directory}}`
+
+- If-else statement
+
+`test {{condition}} && echo "true" || echo "false"`