Tests: map regex capture in rewrite replacement.

[Aayush7352]

Summary

Test that a map variable whose value uses a $1 reference from the map regex pattern does not overwrite the rewrite regex captures when used in a rewrite replacement (ticket #357).

What the test covers

• Default map value (no regex) → rewrite $1 preserved
• Literal map value (no regex) → rewrite $1 preserved
• Map regex with capture, used in rewrite $1 → rewrite $1 is the rewrite regex capture, not the map regex capture
• No rewrite on different URI
• Default value with map regex non-match

Verified

Without the source fix — 2/6 subtests fail (rewrite $1 gets map regex capture, produces 404):

Failed tests:  3-4

With the source fix — 6/6 subtests pass.

Depends on: nginx/nginx#1573.
Related: nginx/nginx#357.