Use http.Read* in testutil

[k1LoW]

Stop io.ReadAll.

[github-actions]

Code Metrics Report

	main (3781590)	#83 (5edf391)	+/-
Coverage	61.9%	62.2%	+0.3%
Code to Test Ratio	1:2.2	1:2.1	-0.0
Test Execution Time	39s	49s	+10s

Details

  |                     | main (3781590) | #83 (5edf391) |  +/-  |
  |---------------------|----------------|---------------|-------|
+ | Coverage            |          61.9% |         62.2% | +0.3% |
  |   Files             |              8 |             8 |     0 |
  |   Lines             |            454 |           452 |    -2 |
  |   Covered           |            281 |           281 |     0 |
- | Code to Test Ratio  |          1:2.2 |         1:2.1 |  -0.0 |
  |   Code              |            775 |           775 |     0 |
- |   Test              |           1685 |          1659 |   -26 |
- | Test Execution Time |            39s |           49s |  +10s |

Code coverage of files in pull request scope (0.0% → 0.0%)

Files	Coverage	+/-
testutil/rcutil.go	0.0%	0.0%

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Reported by octocov