I am still getting the same error - 
When I looked at the log, it is failing on assertion as below - 
1. When process builder did not update the OLI record - 
05:03:06.214 (1422560750)|SOQL_EXECUTE_BEGIN|[27]|Aggregations:0|SELECT id, unitprice FROM OpportunityLineItem WHERE Id = :tmpVar1
05:03:06.214 (1427061323)|SOQL_EXECUTE_END|[27]|Rows:1
05:03:06.214 (1427094355)|HEAP_ALLOCATE|[27]|Bytes:8
05:03:06.214 (1427118604)|HEAP_ALLOCATE|[27]|Bytes:78
05:03:06.214 (1427189495)|HEAP_ALLOCATE|[27]|Bytes:8
05:03:06.214 (1427212773)|HEAP_ALLOCATE|[27]|Bytes:45
05:03:06.214 (1427310436)|HEAP_ALLOCATE|[27]|Bytes:12
05:03:06.214 (1427374222)|VARIABLE_ASSIGNMENT|[27]|oli1|{"s":1,"v":{"Id":"00k2800000841JsAAI","UnitPrice":2.00}}|0x585f6400
05:03:06.214 (1427382881)|STATEMENT_EXECUTE|[28]
05:03:06.214 (1427394432)|HEAP_ALLOCATE|[28]|Bytes:4
05:03:06.214 (1427731260)|SOQL_EXECUTE_BEGIN|[28]|Aggregations:0|SELECT id, unitprice FROM OpportunityLineItem WHERE Id = :tmpVar1
05:03:06.214 (1430402947)|SOQL_EXECUTE_END|[28]|Rows:1
05:03:06.214 (1430431412)|HEAP_ALLOCATE|[28]|Bytes:8
05:03:06.214 (1430456145)|HEAP_ALLOCATE|[28]|Bytes:78
05:03:06.214 (1430489098)|HEAP_ALLOCATE|[28]|Bytes:8
05:03:06.214 (1430544944)|HEAP_ALLOCATE|[28]|Bytes:12
05:03:06.214 (1430587216)|VARIABLE_ASSIGNMENT|[28]|oli2|{"s":1,"v":{"Id":"00k2800000841JtAAI","UnitPrice":2.00}}|0xc362f75
05:03:06.214 (1430593079)|STATEMENT_EXECUTE|[30]
05:03:06.214 (1431045508)|EXCEPTION_THROWN|[30]|System.AssertException: Assertion Failed: Expected: 0.80, Actual: 2.00
05:03:06.214 (1431242264)|HEAP_ALLOCATE|[30]|Bytes:50
05:03:06.214 (1431390494)|FATAL_ERROR|System.AssertException: Assertion Failed: Expected: 0.80, Actual: 2.00



2. When process builder updated the OLI record - 
05:15:28.20 (1723736981)|SOQL_EXECUTE_BEGIN|[27]|Aggregations:0|SELECT id, unitprice FROM OpportunityLineItem WHERE Id = :tmpVar1
05:15:28.20 (1726024469)|SOQL_EXECUTE_END|[27]|Rows:1
05:15:28.20 (1726049180)|HEAP_ALLOCATE|[27]|Bytes:8
05:15:28.20 (1726065403)|HEAP_ALLOCATE|[27]|Bytes:78
05:15:28.20 (1726127621)|HEAP_ALLOCATE|[27]|Bytes:8
05:15:28.20 (1726139079)|HEAP_ALLOCATE|[27]|Bytes:45
05:15:28.20 (1726208221)|HEAP_ALLOCATE|[27]|Bytes:12
05:15:28.20 (1726263859)|VARIABLE_ASSIGNMENT|[27]|oli1|{"s":1,"v":{"Id":"00k2800000841KRAAY","UnitPrice":0.80}}|0x3ac836b7
05:15:28.20 (1726268506)|STATEMENT_EXECUTE|[28]
05:15:28.20 (1726283709)|HEAP_ALLOCATE|[28]|Bytes:4
05:15:28.20 (1726478586)|SOQL_EXECUTE_BEGIN|[28]|Aggregations:0|SELECT id, unitprice FROM OpportunityLineItem WHERE Id = :tmpVar1
05:15:28.20 (1728209872)|SOQL_EXECUTE_END|[28]|Rows:1
05:15:28.20 (1728232795)|HEAP_ALLOCATE|[28]|Bytes:8
05:15:28.20 (1728243642)|HEAP_ALLOCATE|[28]|Bytes:78
05:15:28.20 (1728258269)|HEAP_ALLOCATE|[28]|Bytes:8
05:15:28.20 (1728292073)|HEAP_ALLOCATE|[28]|Bytes:12
05:15:28.20 (1728325130)|VARIABLE_ASSIGNMENT|[28]|oli2|{"s":1,"v":{"Id":"00k2800000841KSAAY","UnitPrice":0.80}}|0x414c31a1
05:15:28.20 (1728329518)|STATEMENT_EXECUTE|[30]
05:15:28.20 (1728489811)|STATEMENT_EXECUTE|[31]
05:15:28.20 (1728587593)|EXCEPTION_THROWN|[31]|System.AssertException: Assertion Failed: Same value: 0.80
05:15:28.20 (1728739045)|HEAP_ALLOCATE|[31]|Bytes:38
05:15:28.20 (1728861562)|FATAL_ERROR|System.AssertException: Assertion Failed: Same value: 0.80

Surprisingly, it is expecting to be different value and at the same time does not allow same values as expected. In second scenario above the unit price changed from 2.0 to 0.8 as expected but still raised assertion. Is there any issue with test code?