fix: Return 404 status on non-existent API calls

[vmrajas]

Motivation

Currently, making an API call to a non-existent path on alpha HTTP server such as "/admin/test" returns status as 200 (OK). As the path does not exist, status should be 404.
Related discuss issue: https://discuss.dgraph.io/t/dgraph-should-return-404-on-non-existent-api-paths/9574

Components affected by this PR
Dgraph alpha http server

Does this PR break backwards compatibility?:
No

Testing
Added two unit tests in dgraph/cmd/alpha/http_test.go .

This Pull Request fixes this.

Fixes
DGRAPH-2335

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This change is 

[CLAassistant]

All committers have signed the CLA.

[arijitAD]

Reviewable status: 0 of 2 files reviewed, 2 unresolved discussions (waiting on @arijitAD, @ashish-goswami, @harshil-goel, @manishrjain, @vmrajas, and @vvbalaji-dgraph)

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dgraph/cmd/alpha/http_test.go, line 941 at r1 (raw file):

	resp, err := client.Do(req)
	require.NoError(t, err)
	require.True(t, resp.StatusCode == 404)

Use require.Equal()

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dgraph/cmd/alpha/http_test.go, line 942 at r1 (raw file):

	require.NoError(t, err)
	require.True(t, resp.StatusCode == 404)
	require.True(t, resp.Status == "404 Not Found")

Use require.Equal()

[vmrajas]

Reviewable status: 0 of 2 files reviewed, 2 unresolved discussions (waiting on @arijitAD, @ashish-goswami, @harshil-goel, @manishrjain, and @vvbalaji-dgraph)

---

dgraph/cmd/alpha/http_test.go, line 941 at r1 (raw file):

Previously, arijitAD (Arijit Das) wrote…

Use require.Equal()

Done.

---

dgraph/cmd/alpha/http_test.go, line 942 at r1 (raw file):

Previously, arijitAD (Arijit Das) wrote…

Use require.Equal()

Done.

[vmrajas]

Reviewable status: 0 of 2 files reviewed, 2 unresolved discussions (waiting on @arijitAD, @ashish-goswami, @harshil-goel, @manishrjain, and @vvbalaji-dgraph)

[arijitAD]

Reviewable status: 0 of 2 files reviewed, 2 unresolved discussions (waiting on @arijitAD, @ashish-goswami, @harshil-goel, @manishrjain, and @vvbalaji-dgraph)