Introduce a minimal debugger for .tcl integration test suite. (#683)

Introduce a break-point function called `bp`, based on the tcl wiki's
minimal debugger.

```tcl
 proc bp {{s {}}} {
    if ![info exists ::bp_skip] {
        set ::bp_skip [list]
    } elseif {[lsearch -exact $::bp_skip $s]>=0} return
    if [catch {info level -1} who] {set who ::}
    while 1 {
        puts -nonewline "$who/$s> "; flush stdout
        gets stdin line
        if {$line=="c"} {puts "continuing.."; break}
        if {$line=="i"} {set line "info locals"}
        catch {uplevel 1 $line} res
        puts $res
    }
 }
```

```
... your test code before break-point
bp 1
... your test code after break-point
```

The `bp 1` will give back the tcl interpreter to the developer, and
allow you to interactively print local variables (through `puts`), run
functions and so forth.

Source: https://wiki.tcl-lang.org/page/A+minimal+debugger

---------

Signed-off-by: Kyle Kim <kimkyle@amazon.com>
Signed-off-by: Madelyn Olson <madelyneolson@gmail.com>
Co-authored-by: Madelyn Olson <madelyneolson@gmail.com>

tests/README.md

@@ -1,6 +1,18 @@
 Valkey Test Suite
 =================
 
+Overview
+--------
+
+Integration tests are written in Tcl, a high-level, general-purpose, interpreted, dynamic programming language [[source](https://wiki.tcl-lang.org/page/What+is+Tcl)].
+`runtest` is the main entrance point for running integration tests.
+For example, to run a single test;
+
+```shell
+./runtest --single unit/your_test_name
+# For additional arguments, you may refer to the `runtest` script itself.
+```
+
 The normal execution mode of the test suite involves starting and manipulating
 local `valkey-server` instances, inspecting process state, log files, etc.
 
@@ -19,6 +31,26 @@ match different external server configurations:
 | `--cluster-mode`     | Run in strict Valkey Cluster compatibility mode. |
 | `--large-memory`     | Enables tests that consume more than 100mb |
 
+Debugging
+---------
+
+You can set a breakpoint and invoke a minimal debugger using the `bp` function.
+
+```
+... your test code before break-point
+bp 1
+... your test code after break-point
+```
+
+The `bp 1` will give back the tcl interpreter to the developer, and allow you to interactively print local variables (through `puts`), run functions and so forth [[source](https://wiki.tcl-lang.org/page/A+minimal+debugger)]. 
+`bp` takes a single argument, which is `1` for the case above, and is used to label a breakpoint with a string.
+Labels are printed out when breakpoints are hit, so you can identify which breakpoint was triggered.
+Breakpoints can be skipped by setting the global variable `::bp_skip`, and by providing the labels you want to skip.
+
+The minimal debugger comes with the following predefined functions.
+* Press `c` to continue past the breakpoint.
+* Press `i` to print local variables.
+
 Tags
 ----
 

tests/support/util.tcl

@@ -1162,3 +1162,25 @@ proc generate_largevalue_test_array {} {
     set largevalue(quicklist) [string repeat "x" 8192]
     return [array get largevalue]
 }
+
+# Breakpoint function, which invokes a minimal debugger.
+# This function can be placed within the desired Tcl tests for debugging purposes.
+# 
+# Arguments:
+# * 's': breakpoint label, which is printed when breakpoints are hit for unique identification.
+# 
+# Source: https://wiki.tcl-lang.org/page/A+minimal+debugger
+proc bp {{s {}}} {
+    if ![info exists ::bp_skip] {
+        set ::bp_skip [list]
+    } elseif {[lsearch -exact $::bp_skip $s]>=0} return
+    if [catch {info level -1} who] {set who ::}
+    while 1 {
+        puts -nonewline "$who/$s> "; flush stdout
+        gets stdin line
+        if {$line=="c"} {puts "continuing.."; break}
+        if {$line=="i"} {set line "info locals"}
+        catch {uplevel 1 $line} res
+        puts $res
+    }
+}