Check prerequisites in install.sh

spec/isMacOS_spec.sh

@@ -0,0 +1,40 @@
+Describe 'isMacOS'
+  Include ./install.sh
+  uname() {  [ "${1}" = -s ] && echo Darwin }
+
+  It 'returns failure if sw_vers is not installed'
+    sw_vers() { echo "zsh: command not found: sw_vers" >&2; return 127 }
+    When call isMacOS
+    The output should eq ''
+    The status should be failure
+  End
+
+  It 'returns failure if os version is below 10.13'
+    sw_vers() { echo "1.1" }
+    When call isMacOS
+    The output should eq ''
+    The status should be failure
+  End
+
+  It 'returns success if os version is 10.13'
+    sw_vers() { echo "10.13" }
+    When call isMacOS
+    The output should eq ''
+    The status should be success
+  End
+
+  It 'returns success if os version is above 10.13'
+    sw_vers() { echo "17.04" }
+    When call isMacOS
+    The output should eq ''
+    The status should be success
+  End
+
+  It 'returns failure if kernel name is not Darwin'
+    sw_vers() { echo "10.13" }
+    uname() {  [ "${1}" = -s ] && echo Linux }
+    When call isMacOS
+    The output should eq ''
+    The status should be failure
+  End
+End