tests/vm/tslow_tables.nim is flaky (at least on windows)

[timotheecour]

history: https://dev.azure.com/nim-lang/Nim/_test/analytics?definitionId=1&contextType=build

windows https://dev.azure.com/nim-lang/255dfe86-e590-40bb-a8a2-3c0295ebdeb1/_apis/build/builds/5251/logs/66

2020-05-13T04:49:18.2085579Z FAIL: tests/vm/tslow_tables.nim C
2020-05-13T04:49:18.2086103Z Test "tests\vm\tslow_tables.nim" in category "vm"
2020-05-13T04:49:18.2086612Z Failure: reTimeout
2020-05-13T04:49:18.2086978Z Expected:
2020-05-13T04:49:18.2087295Z create
2020-05-13T04:49:18.2087585Z search
2020-05-13T04:49:18.2087813Z done
2020-05-13T04:49:18.2087974Z 
2020-05-13T04:49:18.2088265Z Gotten:
2020-05-13T04:49:18.2088659Z Hint: used config file 'd:\a\1\s\config\nim.cfg' [Conf]
2020-05-13T04:49:18.2089238Z Hint: used config file 'd:\a\1\s\config\config.nims' [Conf]
2020-05-13T04:49:18.2089766Z Hint: used config file 'd:\a\1\s\tests\config.nims' [Conf]
2020-05-13T04:49:18.2090224Z create
2020-05-13T04:49:18.2090462Z search
2020-05-13T04:49:18.2090754Z done
2020-05-13T04:49:18.2091141Z CC: stdlib_io.nim
2020-05-13T04:49:18.2091835Z CC: stdlib_system.nim
2020-05-13T04:49:18.2092224Z CC: tslow_tables.nim
2020-05-13T04:49:18.2092642Z Hint:  [Link]
2020-05-13T04:49:18.2093825Z Hint: 71317 LOC; 5.337 sec; 181.25MiB peakmem; Debug build; proj: tslow_tables; out: tslow_tables.exe [SuccessX]
2020-05-13T04:49:18.2494524Z 
2020-05-13T04:49:18.2495276Z JOINED: tests/vm/tslurp.nim C

again here https://dev.azure.com/nim-lang/Nim/_build/results?buildId=5280&view=logs&jobId=1effb61f-3820-5ca5-606b-caa183b197b5&j=1effb61f-3820-5ca5-606b-caa183b197b5&t=f5fa2c17-e33f-5e55-c057-4657fef6c5dd

FAIL: tests/vm/tslow_tables.nim C
Test "tests\vm\tslow_tables.nim" in category "vm"
Failure: reTimeout

[timotheecour]

and again:
https://dev.azure.com/nim-lang/255dfe86-e590-40bb-a8a2-3c0295ebdeb1/_apis/build/builds/5564/logs/66

2020-05-26T00:57:11.6296561Z PASS: tests/vm/tsimpleglobals.nim C                                ( 5.79 sec)
2020-05-26T00:57:11.6310046Z FAIL: tests/vm/tslow_tables.nim C
2020-05-26T00:57:12.3990885Z Test "tests\vm\tslow_tables.nim" in category "vm"
2020-05-26T00:57:12.5821608Z Failure: reTimeout
2020-05-26T00:57:12.9718919Z Expected:
2020-05-26T00:57:13.0106702Z create
2020-05-26T00:57:13.0123480Z search
2020-05-26T00:57:13.0125258Z done
2020-05-26T00:57:13.0127670Z 
2020-05-26T00:57:13.0129123Z Gotten:
2020-05-26T00:57:13.1395070Z Hint: used config file 'd:\a\1\s\config\nim.cfg' [Conf]
2020-05-26T00:57:13.1395935Z Hint: used config file 'd:\a\1\s\config\config.nims' [Conf]
2020-05-26T00:57:13.1396556Z Hint: used config file 'd:\a\1\s\tests\config.nims' [Conf]
2020-05-26T00:57:13.1397084Z create
2020-05-26T00:57:13.1397326Z search
2020-05-26T00:57:13.1397618Z done
2020-05-26T00:57:13.1397905Z CC: stdlib_io.nim
2020-05-26T00:57:13.1398302Z CC: stdlib_system.nim
2020-05-26T00:57:13.1398875Z CC: tslow_tables.nim
2020-05-26T00:57:13.1399256Z Hint:  [Link]
2020-05-26T00:57:13.1399930Z Hint: 70019 lines; 4.954s; 181.559MiB peakmem; Debug build; proj: tslow_tables; out: tslow_tables.exe [SuccessX]
2020-05-26T00:57:13.1400478Z 
2020-05-26T00:57:13.1400854Z JOINED: tests/vm/tslurp.nim C