Use string.format to fix floating point error in test output

[sparr]

Floating point error causes some test benchmark times to have a bunch of trailing 9s or 0s before some noise at the end.

Before:

诊断文件耗时：  0.0048709999999978      file:///Users/sparr/src/lua-language-server/script/SDBMHash.lua
诊断文件耗时：  0.545929        file:///Users/sparr/src/lua-language-server/script/await.lua
诊断文件耗时：  0.0079290000000007      file:///Users/sparr/src/lua-language-server/script/brave/log.lua

After:

诊断文件耗时：  0.004871        file:///Users/sparr/src/lua-language-server/script/SDBMHash.lua
诊断文件耗时：  0.545929        file:///Users/sparr/src/lua-language-server/script/await.lua
诊断文件耗时：  0.007929        file:///Users/sparr/src/lua-language-server/script/brave/log.lua

[sumneko]

Thank you!