Add a test suite under Tests

[TetzkatLipHoka]

Nine console test programs plus a script that builds and runs them with every Delphi installation it finds, for Win32 and Win64. Nothing in FastMM5.pas changes - this is purely additive, and if you never run it, it costs nothing.

The motivation is #102: the corruption scan silently stopped seeing small debug blocks, and it stayed unnoticed because there was nothing that would have run the same check across all three size classes after a change. FastMM5Test_ScanCoverage is exactly that check, and it fails on the affected commits and passes on either side of them.

How it works

No test framework. Each program is a plain console application that prints one line per check and exits with 0 when everything passed, otherwise with the number of failed checks. RunTests.ps1 builds and runs them all and exits with the number of failed runs, so it drops straight into a CI step:

pwsh -File Tests\RunTests.ps1                  # every configured compiler, Win32 and Win64
pwsh -File Tests\RunTests.ps1 -Only D13.1      # a single compiler
pwsh -File Tests\RunTests.ps1 -Quick           # shorter stress runs

The $Compilers table at the top of the script is the only thing that needs adjusting for a different machine.

What is covered

Program	What it covers
FastMM5Test_DebugMode	Entering and leaving debug mode, allocate/write/realloc/free in both modes, allocated bytes returning to the starting value.
FastMM5Test_SizeClasses	Every size class from 1 byte to 2 MB plus reallocations across the boundaries, each verified with a fill pattern.
FastMM5Test_UsagePerSizeClass	Churn per size class: nothing may stay allocated, and the committed address space must not keep growing from phase to phase.
FastMM5Test_ModeTransition	The Begin/End contract from #85, including the case that a failed Begin still has to be balanced.
FastMM5Test_DoubleFree	#73: the second free is rejected and leaves the pending free list intact (no self cycle). A walker thread sleeping on the block under test forces the pending free path deterministically.
FastMM5Test_ScanCoverage	#102: corrupted header checksum, overrun into the footer, and write into a freed block - across small, medium and large.
FastMM5Test_ScanRace	The other direction: threads churning small debug blocks while a scanner runs must not produce a false positive or a crash.
FastMM5Test_ScanHeaderBounds	Corrupts UserSize / StackTraceEntryCount, i.e. the fields that decide where the scan reads; each case must give a clean report rather than an A/V.
FastMM5Test_MultiThreadStress	Multithreaded stress with optional cross thread frees through a lock free mailbox; content integrity plus closing balance.

FastMM_TestUtils.pas holds the assertions and the exit code convention. It also clears FastMM_MessageBoxEvents and FastMM_LogToFileEvents for the duration of a run, since several tests corrupt blocks on purpose and a modal dialog would hang an unattended run.

Testing

36 of 36 runs pass: Delphi 10 Seattle and Delphi 13.1, each Win32 and Win64.

Two notes

The Tests/README.md records two things that cost me time and would otherwise be rediscovered by whoever writes the next test: corrupting a freed small block is not observable (raising the report allocates the exception object, which is handed exactly that block, so the process dies before any handler runs - use medium or large blocks there), and a corruption test that happens to use a large block proves nothing about the small block path, which is how #102 stayed hidden.

The sources carry {$if CompilerVersion >= ...} guards in a few places so the same files also build on older compilers in a fork. They are inert on XE3 and later; happy to strip them if you would rather not carry them.