extra_test.go: Add test timeout multiplier environment variable

[piggynl]

Hi,

Currently some performance tests in extra_test.go are failing in low-performance environments (QEMU user mode emulation in my case).

=== RUN   TestDeepNestedLabelPerformance
    extra_test.go:112: Parsing deep nested labels took more 5 secs
--- FAIL: TestDeepNestedLabelPerformance (25.33s)
=== RUN   TestManyProcessingInstructionPerformance
    extra_test.go:132: Parsing processing instructions took more 5 secs
--- FAIL: TestManyProcessingInstructionPerformance (14.11s)
=== RUN   TestManyCDATAPerformance
    extra_test.go:152: Parsing processing instructions took more 5 secs
--- FAIL: TestManyCDATAPerformance (33.70s)
=== RUN   TestManyDeclPerformance
    extra_test.go:172: Parsing processing instructions took more 5 secs
--- FAIL: TestManyDeclPerformance (16.28s)
=== RUN   TestManyCommentPerformance
    extra_test.go:192: Parsing processing instructions took more 5 secs
--- FAIL: TestManyCommentPerformance (40.88s)

I think reading a timeout multiplier from environment variable is a reasonable way to test goldmark in those low-performance environments without affecting the tests in normal environments.

Thank you for looking into this PR!

[stale]

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