Fixed unit tests

[Fiddlekins]

• Fixed issue where the tests make repeat calls to vm.readLine, which retains the cursor between calls and thus returns empty strings after the first call. Solution was reading entire file each time and extracting the first line from that manually
• Fixed mkdir crashing tests when running from windows shell, now it silently fails
• Removed unused TEMP_ENV_VAR constant from GasSnapshot
• Now uses vm cheatcode to parse string to uint. This also seems to fix issue where write and check modes expand memory by different amounts, which results in written gas snaps not matching the values determined during check mode when doing multiple snapshots in a single test. (testAddTwice was failing previously)
• updated expected value in testCheckManySstoreFails, presumably this has reduced due to optimisation gains in newer solidity versions. Current value was determined using solc 8.19.0