Added the test cases for adding assignee in issue on jira through slash command.

data/test-cases/plugins/jira/jira-server/jira-assign/Assign_user.md

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+---
+# (Required) Ensure all values are filled up
+name: "Assign a Jira issue to a user through slash command."
+status: Active
+priority: Normal
+folder: Jira-assign
+authors: "@arush-vashishtha"
+team_ownership: []
+priority_p1_to_p4: P2 - Core Functions (Do core functions work?)
+
+# (Optional)
+location: null
+component: null
+tags: []
+labels: []
+tested_by_contributor: null
+
+# (Optional) Test type and tools
+cypress: null
+detox: null
+mmctl: null
+playwright: null
+rainforest: []
+manual_test_environments: []
+
+# Do not change
+id: null
+key: null
+created_on: null
+last_updated: null
+case_hashed: null
+steps_hashed: null
+---
+
+**Step 1**
+
+1. Run the `/jira assign` and add the desired issue key in the slash command in the channel on MM.
+2. Add the MM username in the <assignee> in above command followed by a `@` when the user's MM account is connected to jira account.
+
+**Step 2**
+
+1. Run the `/jira assign` and add the desired issue key in the slash command in the channel on MM.
+2. Add the MM username in the <assignee> in above command not followed by a `@` when the user's MM account is connected or disconnected to jira account.
+
+**Step 3**
+
+1. Run the `/jira assign` and add the desired issue key in the slash command in the channel on MM.
+2. Add the MM username in the <assignee> in above command when the user's MM account is disconnected to jira account.
+
+**Step 4**
+
+1. Run the `/jira assign` and add the desired issue key in the slash command in the channel on MM.
+2. Add the jira username in the <assignee> in above command not followed by a `@` when the user's MM account is connected or disconnected to their jira account.
+
+**Expected**
+
+The user should get suggestions while entering MM user name and the assginee should get added to the issue on Jira.
+After step 2, the user should not get any suggestion while entering MM user name and they should receive the error message that no assignee found for MM user name.
+After step 3, the user with MM user name should not get assigned to the issue on jira.
+After step 4, the user with jira user name should get assigned to the issue on jira.