Robot flake fixes 🤖❄️ (#69)

* use random port, always sleep hard before starting editor test

* fix up some robot library usage

* robot variable docs

Author: bollwyvl

atest/01_Editor.robot

@@ -91,7 +91,8 @@ Editor Should Jump To Definition
     Set Tags    feature:jump-to-definition
     ${sel} =    Set Variable If    "${symbol}".startswith(("xpath", "css"))    ${symbol}    xpath:(//span[@role="presentation"][contains(., "${symbol}")])[last()]
     Mouse Over    ${sel}
-    Run Keyword If    "${OS}" == "Windows"    Sleep    10s
+    Sleep    10s
+    Mouse Over    ${sel}
     Wait Until Keyword Succeeds    10 x    0.1 s    Click Element    ${sel}
     Wait Until Keyword Succeeds    10 x    0.1 s    Open Context Menu    ${sel}
     ${cursor} =    Measure Cursor Position

atest/Keywords.robot

@@ -4,10 +4,14 @@ Library   SeleniumLibrary
 Library   OperatingSystem
 Library   Process
 Library   String
+Library   ./Ports.py
 
 
 *** Keywords ***
 Setup Server and Browser
+    ${port} =  Get Unused Port
+    Set Global Variable    ${PORT}    ${port}
+    Set Global Variable    ${URL}  http://localhost:${PORT}${BASE}
     ${accel} =  Evaluate    "COMMAND" if "${OS}" == "Darwin" else "CTRL"
     Set Global Variable  ${ACCEL}  ${accel}
     ${token} =   Generate Random String

atest/Ports.py

@@ -0,0 +1,16 @@
+""" get a random port
+"""
+import socket
+
+
+def get_unused_port():
+    """ Get an unused port by trying to listen to any random port.
+
+        Probably could introduce race conditions if inside a tight loop.
+    """
+    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
+    sock.bind(("localhost", 0))
+    sock.listen(1)
+    port = sock.getsockname()[1]
+    sock.close()
+    return port

atest/Variables.robot

@@ -1,6 +1,8 @@
 *** Variables ***
-${PORT}  18888
 ${SPLASH}  id:jupyterlab-splash
+
+# to help catch hard-coded paths
 ${BASE}   /@est/
-${URL}  http://localhost:${PORT}${BASE}
+
+# override with `python scripts/atest.py --variable HEADLESS:0`
 ${HEADLESS}  1