Fix timing issue in sub-second expires test (#9821)

The `PEXPIRE/PSETEX/PEXPIREAT can set sub-second expires` test is
a very time sensitive test, it used to occasionally fail on MacOS.

It will perform there internal tests in a loop, as long as one
fails, it will try to excute again in the next loop.

oranagra suggested that we can split it into three individual tests,
so that if one fails, we do not need to retry the others. And maybe
it will increase the chances of success dramatically.

Each is executed 500 times, and the number of retries is collected:
```
PSETEX, total: 500, sum: 745, min: 0, max: 13, avg: 1.49

PEXPIRE, total: 500, sum: 575, min: 0, max: 16, avg: 1.15

PEXPIREAT, total: 500, sum: 0, min: 0, max: 0, avg: 0.0

ALL(old_way), total: 500, sum: 8090, min: 0, max: 138, avg: 16.18
```

And we can see the threshold is very low.
Splitting the test also makes the code better to maintain.

Co-authored-by: Oran Agra <oran@redislabs.com>

tests/unit/expire.tcl

@@ -91,20 +91,27 @@ start_server {tags {"expire"}} {
         list $a $b
     } {somevalue {}}
 
-    test {PEXPIRE/PSETEX/PEXPIREAT can set sub-second expires} {
-        # This test is very likely to do a false positive if the
-        # server is under pressure, so if it does not work give it a few more
-        # chances.
+    test "PSETEX can set sub-second expires" {
+        # This test is very likely to do a false positive if the server is
+        # under pressure, so if it does not work give it a few more chances.
         for {set j 0} {$j < 50} {incr j} {
             r del x
-            r del y
-            r del z
             r psetex x 100 somevalue
             after 80
             set a [r get x]
             after 120
             set b [r get x]
 
+            if {$a eq {somevalue} && $b eq {}} break
+        }
+        if {$::verbose} { puts "PSETEX sub-second expire test attempts: $j" }
+        list $a $b
+    } {somevalue {}}
+
+    test "PEXPIRE can set sub-second expires" {
+        # This test is very likely to do a false positive if the server is
+        # under pressure, so if it does not work give it a few more chances.
+        for {set j 0} {$j < 50} {incr j} {
             r set x somevalue
             r pexpire x 100
             after 80
@@ -112,6 +119,16 @@ start_server {tags {"expire"}} {
             after 120
             set d [r get x]
 
+            if {$c eq {somevalue} && $d eq {}} break
+        }
+        if {$::verbose} { puts "PEXPIRE sub-second expire test attempts: $j" }
+        list $c $d
+    } {somevalue {}}
+
+    test "PEXPIREAT can set sub-second expires" {
+        # This test is very likely to do a false positive if the server is
+        # under pressure, so if it does not work give it a few more chances.
+        for {set j 0} {$j < 50} {incr j} {
             r set x somevalue
             set now [r time]
             r pexpireat x [expr ([lindex $now 0]*1000)+([lindex $now 1]/1000)+200]
@@ -120,15 +137,11 @@ start_server {tags {"expire"}} {
             after 220
             set f [r get x]
 
-            if {$a eq {somevalue} && $b eq {} &&
-                $c eq {somevalue} && $d eq {} &&
-                $e eq {somevalue} && $f eq {}} break
+            if {$e eq {somevalue} && $f eq {}} break
         }
-        if {$::verbose} {
-            puts "sub-second expire test attempts: $j"
-        }
-        list $a $b $c $d $e $f
-    } {somevalue {} somevalue {} somevalue {}}
+        if {$::verbose} { puts "PEXPIREAT sub-second expire test attempts: $j" }
+        list $e $f
+    } {somevalue {}}
 
     test {TTL returns time to live in seconds} {
         r del x